How do I know when travel is no longer possible?

The obvious answer is no, you don't go, and keep in mind the pressure you're getting is from (I assume) people who have no real idea of her actual condition. I would give their pressuring absolutely no credence whatsoever.

I myself won't travel much anymore, and that's with no deficits of any kind save being 80 years old and having to pack the medicine cabinet along with me (kidding). It just becomes more difficult to be out of my comfortable environment, and I will now only travel to those with whom I am perfectly and entirely comfortable, say my daughter several states away (scheduled to fly out in a month and dreading the whole "airplane" thing.
However, my daughter's mother in law is STILL traveling (while admitting it is harder) in her mid 90s.
So this is a question as individual as your own fingerprint.
I wouldn't be going to this reunion and I wouldn't be enjoying it if I DID go. What about your wife? And more importantly, what about you.
It wouldln't take me to seconds to answer. My answer would be no, and my response to queries would be "This is another loss and I am sad about it, but grateful for having had it, and willing to know my limitations now".
I sure wish you the best in making your decision and being at peace with it. My answer would be "no". Others would definitely try it. Best out to you.

If your wife needs to have a live-in caregiver because her Alzheimer's is far enough advanced to make this necessary, she cannot attend her family reunion.
Forcing her to be driven an hour away to a place she will not recognize now to be bombarded by people she will also very likely not recognize, will be a traumatic experience for her.
It may even cause setbacks for her too. Like not recognizing her home when you get back. Ot total incontinence and not being able to be toileted. Or not being able to walk up the stairs even with help. Sometimes these setbacks can be permanent.
If your wife's family puts pressure on you to attend, tell them that nothing is stopping any one of them from coming to your house if they want to visit your wife. The reunion will be too traumatic for her.
Think of the planning. What happens if she's incontinent on the car ride? Where would you change her? A gas station bathroom? What if it's a mess? What then?
What if she gets upset and has a meltdown because she wants to go home? You're an hour away from home. Don't bring her. If anyone puts the heat on you to go to the family reunion ask them the questions I am asking you here. Also ask them to consider what having her there would be like for everyone else attenting. Would everyone be comfortable eating their foods when someone craps themselves at the table? Would they be put off watching your wife eat or being fed?
I was a caregiver mostly to elderly for almost 25 years. I can say that I have never been able to eat a meal with a dementia client because it made me sick to watch them slopping food down themselves, smacking with their mouths full, and often burping and farting at the same time. I always encouraged self-feeding because that's a form of independence. I always cleaned up any meeses and that was fine with me. I often had to spoon-feed and that was part of the job and fine too. I could never eat anything with them myself though.
Ask your wife's family if everyone would be comfortable with this at the party. Tell them that you're keeping her home as much out of a consideration for them as for her.

When I start getting "pressure" to take my husband places I know will be too difficult for me here is my answer... I am the one who will pay the consequences and I simply am not willing to risk those consequences. Others can not understand what you deal with 24/7 and can not envision the issues you have to consider.

Here are issues I consider whether it is 10 minutes away or an hour...

1. Assisting in toileting or changing briefs in a familiar environment... will there be a family restroom or will there be someone to help instead and will it be clean. Especially important if a stop is needed along the way.

2. Being sure I bring appropriate foods since my husband has swallowing difficulty. Being around so many great foods, I may constantly have to say are not good choices. Having enough extra clothing for accidents. Having enough supplies in general for the unexpected.

3. Simply not enjoying conversation because of the noise level especially while trying to focus on his safety when he is drawn to the food table or get may lost in the crowd. He is hard to understand and everyone looks at me to interpret... and I often can't even understand. I find myself trying to protect his dignity.

4. Will I get enough enjoyment for all the effort I will have to put into getting there and back. On certain instances, I put my enjoyment aside if I feel it is important for him and focus on helping him experience it. I have had to miss many events I really wanted to go as well. So again, I always ask if am I willing to pay the consequences for limited or no benefits... sometimes yes and sometimes no.

5. Settling back at home afterwards can be a challenge after all the outside stimulation (good and bad) you both experience especially after a long one day trip.

6. Will the COVID numbers be low enough that I am willing to take the risk.

My sons tell me I worry too much about what can go wrong and I can not convince them that only I know what often happens good or bad. I already know what happens at home and this is what I base my decision and considering the unexpected seals that decision. I have to envision dealing with those same issues on a trip there and back as well as while we are there. If I feel I can handle those issues away from home in that situation then I agree to go. Sometimes we both need a break getting out and it is worth the consequences and sometimes we both need a break by staying home. Do not let them pressure you to do what you are not comfortable.

All that being said, have you considered having a sitter and you go alone? You are probably very important to her family and they would love to see how you are doing. You can bring photos or videos of her to share to let them see how she is doing. It might be the break you need and help her family understand how vulnerable she is now. They may decide it is important to come and visit her so be prepared to offer guidelines if you want them coming.... no more than XX people at a time, no longer than XX amount of time, and any other considerations you would appreciate. I have had to remind others to not bring foods without letting me know... even if they are for me because then I have to hide them.

Even for the cognizant, family reunions can be extremely stressful. Add to that that she might not remember these people at their present age.

If you’re comfortable, invite participants to come by in small groups instead.

Now. Travel is no longer possible now, considering the list of conditions your wife is afflicted with. It won't be fun or enjoyable for either one of you, let's face it. Familiarity is what she needs, and a shrunken down world where she's happy knowing where everything is.

What I find is, people do not understand dementia, nor do they want to understand dementia. All they understand is, there's a Big Family Reunion Coming Up and We Expect Both of You To Be There. Period. So if I were you, I'd tell them you're both sick with the flu or worse yet, diarrhea, and can't possibly make it, much as you'd LOVE to. Nobody eveeeeeeer questions diarrhea. They'll leave you alone and understand totally why you won't be in attendance at the family reunion.

It's a way of making YOUR life easier. It's all about you and your wife now, and how to simplify things. Not how to pacify others who don't get it, or offer you no help in caring for this dreadful disease your dear wife is afflicted with.

God bless you and help you through this difficult journey.

I’m in agreement with others that a full reunion will be overwhelming.
Is the schedule for the reunion such that there is an “early admission” or a “patron party” type of opportunity to have a much much smaller group to get together the day before the big reunion? If so, I’d try to plan something.

So if you really really want to do this, I’d talk with whomever in her fam who is the organizer to see if you can plan a get together ahead of the event. It’s catered and at the reunion place if it’s at a resort or a handicapped accessible room at a restaurant close to where the reunion is happening. And you bring your brides caregiver to the event to help your wife navigate the evening. ((Do ask the caregiver if she thinks she may need a second set of hands too.)) Keep it short, like 3 hours max….. have this in the contract at the place too. To get folks to leave, if you anticipate an issue with this, you have swag bags done which you pay 1 or 2 servers to give to her family AS THEY LEAVE.

By swag bags, I mean a small gift bag that could have couple of old family photos and a maybe something else that’s very much your wife. What I did for a luncheon for the anniversary of my moms death, was a bag (You can do Dollar Tree) with a terra cotta planter inside it with an assortment of old family photos and seed packs of ranunculas (moms favorite). My family is really really bad on doing a long goodbye as someone or two will settle out at a seat a bar or outside with no ice needed adult beverages at anyones home. So I paid 2 servers to give the bags outside on the way to the parking lot.

Now if this seems too too hard to do, perhaps have a religious service for the reunion weekend at the old family church / synagogue in memorial for all the members of her family who have passed. Churches do this and it gets written up in the calendar so friends of the family will show up for the mass / service. I did this too and it was amazing as to who reads the church bulletin and goes!. If her old church has a “baby & toddler” room (glass wall between main part of the church and this room) you can a$k for pews set aside for her (can get florals done too just in this room)so you and her & her caregiver & her family in the room, this way you can visit a bit before and after service. It’s a controlled environment - like a meal would be - which is imo kinda the key to being able to have this happen with minimal issues. Good luck!

You just tell them that her Dementia has progressed where she does not to well in crowds. That it would be too overwhelming. The drive alone could cause her problems. One member's LO tried to get out of the car when it was moving.

My Mom lasted about an hour when we went out. In Church everyone coming up to her and giving her hugs overwhelmed her. The incontinence is another thing. I too would not want to clean her up in a strange place. How can you enjoy a party when you are watching your wife and she probably won't enjoy it or probably know anyone. I agree, leave her home with the aid and u go and enjoy.

Don’t do it. It will be overwhelming and agitating for her.

Your wife would not do good with large crowds and lots of noise and commotion, as people with any of the dementias don't do well when their routine is disrupted, so honestly it's best that you keep her home in her comfort zone. I'm sure her family will understand.
And perhaps since you have a live in caregiver, that person can stay with your wife so you can at least you could go and enjoy a break from caregiving.

I suggest that you do not bring her.
If you wish to go in her stead go but leave her with the caregiver.
If you really truly think she should go bring the caregiver with you. Just know that she will probably want to go home within the first 20 minutes if she lasts that long.
BUT..I think she would be happier at home.
If there are relatives that have not seen her and wish to as you say it is 1 hour away they can come visit her if they truly want to see her.
I think if the family is aware of her diagnosis then they should understand her not attending. (and if they don't not sure I would want them around)

Terrible idea to take an advanced dementia patient to a loud, unfamiliar and confusing environment.

Perhaps you could FaceTime with the individually during the event?

I had this problem several years ago. I decided not to attend our reunion because it would be too much to clean my husband in someone’s home. So, we stayed home and joined the reunion via zoom.