Not sure if I should try to do this trip with MIL...

No, not a good idea. I live on an Island and find Day trips on the Ferry, even with my car to be exhausting. I could not imagine doing it by bus with someone with dementia.

Is this your nephew who is graduating or your son?

Take a video or ask the school if they will be doing a live broadcast of the graduation. I know my kid's high school did a live stream of the graduation as does the university I attend.

She will be much happier in familiar surroundings and you will be able to enjoy the event if she stays home.

No, if you want to enjoy the day with your son, leave her at home.

Is this an indoor or outdoor ceremony? If indoor she would likely get too cold. If outdoor the heat too much. What is the size of the graduating class? Too many variables to consider to make this trip pleasant for her and you.

Leave her home.

Resounding NO
Take a video if you wish.
A good day for someone with dementia is a day that is just like yesterday and the day before that and the day before that. Consistency from the time she gets up and the routine that follows. (same time for breakfast maybe even the same foods through to dinner and bed time and the same people) To get her up early get her into a car, onto a bus will be confusing at best and frightening as well. Not to mention all the noise all the people.
Do yourself a favor enjoy your son's graduation. Let your son enjoy his graduation this is his day. He can go visit grandma later with his diploma.

Goodness no. It’s your Grandson’s day.

It may be detrimental for your MIL to cope with the travel. I would not do it.

What you are describing has many possibilities of turning into a nightmare at the time, and (even worse) leading to her decline and death. That would not be memory your son wants to keep. MIL won't remember it all anyway. Get your son to dress up in the graduating gear with diploma in hand next time he visits, and take a photo of him with her then. If you want, get someone to come in and make a little speech. Everyone can remember that very happily as your 'family graduation'.

I would say that this trip has the potential to be a total nightmare which is not the graduation day everyone like to remember. For a person with dementia keeping a schedule and being in familiar situations is crucial. You will be in transit and/or waiting around for 4 1/2 - 5 hours BEFORE the ceremony even begins. You say that your MIL is "repetitive and demanding" so I can't imagine at this point she is going to willingly sit still and quiet to attend the ceremony which could last a couple of hours. So, you will have to remove her and you will miss the ceremony. And you have no easy way to get home at the end of all of this - back on the bus and ferry for another 2 1/2 hours. Please, please for everyone's sake, take lots of pictures, enjoy the day with your son and assure him that grandmother is proud of him and that she will enjoy all the picture and stories of his special day when you tell her about it.

Personally, I would not risk it. Go and enjoy it yourself. Share photos afterwards if you like.

I would not take her. Especially if she has an incontinence problem. For a Dementia patient it will be too tiring. My Mom was ready to go home after an hr. If this is your son, you should be able to enjoy his graduation without worry about how she will act.

Enjoy the day, it only happens once.

So for me part of it depends on how important it is to the graduating grandson and his parent that she be there. My niece is graduating in June from high school and while I live 4-5 hr drive from my mom and brother I will be going and taking charge of mom attending because it's important to all three of them (niece, brother and Mom) though I know it wont be easy. My brother made the 5 hr drive with Mom for my sons college graduation and we both flew her to the west coast for my nephews high school graduation last year (that was a trip!). It feels good to me to be able to do the same for my brother and his daughter as well as my mom. That said if indeed some of these things are the same for you, first having your son to help is heaven and I would urge you to reconsider driving your own car rather than going the foot traffic route on the ferry and then bus. I understand the hassle of waiting for the ferry etc but my guess is it wont add a whole lot of time and will be far less stressful for mom and you if you aren't in "public" the whole time and without a private place. Given the early hour for mom she can sleep in the back seat, she can nap during the day even (find shade) and she you all have family privacy. If the timing is that difficult perhaps think about going the night before and renting a room so you aren't traveling the morning of graduation or something.

Few questions: How old is MIL? any ailments or health issues to deal with? Overall it sounds like a long trip and if she is mentally exhausting to deal with already maybe reconsider, than again I don't know how important it is for her to just be present for her grandson, she has dementia and will not recall the event but it could be important for her grandson. Several factors here. Family discussion with all parties involved to see what they think. Wishing you the best in your situation :)