Bringing mom home for the night. Would that confuse her?

No. She needs routine and it ultimately does more harm than good.

I would not have a sleepover. Her idea of a 'sleepover' is sleeping in her bed at the Assisted Living facility. Short excursions are fine. I've found that an hour drive in the country is plenty enough. Save lunch for another visit.
Memories are more important now than mini-vacation which might just be a heavy burden to bear or share.
Good thing you and your mom can finally get together.

What I found at times, was that I had these grand plans to have fun with my mom, but she couldn't just enjoy the moment. Her vascular dementia ruined it by her wondering other things (such as where did she leave her car or the baby?). I finally had to cut the activity short and take her back to MC. It's worth a try, but don't get too disappointed if she doesn't understand the significance of the activity. Be prepared to cut the night short. I hope it does work out though! It could be a sweet time.

Maybe it would be a better idea for you pick mom up for a few hours to have lunch at your house and a visit then bring her back to the AL. I wouldn't start with sleepovers though. Not if she had dementia and is used to the AL she lives in and used to the routines there. Especially the bedtime ones. She's been confined to her facility for a long time because of Covid. I think a sleepover somewhere else could easily overwhelm her and become a nightmare for you. Start small. A few hours out here and there weather permitting, and see how it goes. Then maybe a sleepover will be a workable possibility for you and mom.

While basically I agree that it isn't wise to take her out of her normal routine, I also think that giving this a try isn't a terrible idea if you have your heart set on it.
You could pose it as "we are having a vacation" and give it a try. You know her much better than we do. If the day out is not too much for her, she will likely be relaxed at home, and ready to rest. Only you can really answer this. If she becomes agitated is there someone at her ALF to accept her in? I know at my bro's place there was always one staff person in each cottage at night.

I do not think a "sleepover" is a good idea.
Very good possibility she will want to "go home" once you start trying to get her ready for bed. To move her for 1 night might be very confusing for her. She knows where her room is, her bathroom, her closet, dresser.....
Also keep in mind that your day out will be exhausting for her. What is "fun" for you might not be fun for her. Take her out, do a bit of shopping, maybe a walk in a park, have a nice lunch then bring her home (her home not yours). Next week take her out again. You do not have to put a years worth of outings in 1 day. Just be glad that she is vaccinated.
PLEASE remember that you both still need to wear masks, you still need to social distance and handwashing is still important. While the vaccine is a great start there are ever changing variants and there will always be a % of the population that will not or can not vaccinate and a % that will not wear masks. So precautions are still very necessary.

As we age we tend to look forward to the comfort of our own homes and routines. Not as exciting to have a pajama party as in the past. We miss our pillow, our mattress, our cup of hot tea and the news, etc. I agree with cwillie that sleeping over will probably create confusion in her rather than a pleasant experience. She will already probably be exhausted by her day out. I hope you have some sweet time together!

Sometimes we get an idea that our loved ones are missing out on life, but in reality they are content as they are. I don't think that a "sleepover" is a good idea, older people in general do better with familiar surroundings and routines, for those with dementia that is X10.