This is a difficult question and one that I think about sometimes for when I'll eventually have my own kids. I've been ABDL since right when I finished potty training, so 3.5, so I'm in the same boat as you.
A positive thing about your (and my) situation is that ABDL isn't a kink thing (or so it seems for you), since it started so early. I would NEVER give advice to ABDL (kink) parents to tell them for their kids to wear diapers. Pushing sexual things on children is a huge no-no.
It is indeed a tough spot to be in, because if you deny them to wear diapers, it's only going to come back later and a lot stronger of a wanting/urge, along with distrust and bad feelings towards the person who told them they can't. As you stated, denying them downright causes many emotional and mental problems for many years, which many of us know firsthand.
I think the best option would be to have a light conversation with her first about why she likes to wear diapers and set a stage where both of you are comfortable talking about the subject. Closing off on either side hurts both sides.
In my opinion though, if I could go back 12+ years and be in the same situation again (I'm also on the spectrum), I would've taken the "yes" option in a heartbeat. Not only would it have made me happier, saving stress, identity problems, and gloom of not being able to be myself, but it would've made my relationship with my parents a lot closer than it was. It was on my mind all the time since I was about 5, but I was too embarrassed to tell them because I was scared what they would think/do about it, which means I held onto it until I was 18. It was the best day of my life to get it off my chest, but the worst 15 years of holding it in.
In conclusion, I believe in free choice in this branch of parenting. I think your daughter wants diapers for the same reason as many of us at that age, being the feelings of safety and comfort, and if she wants to wear (as long as you're not financially straining and you're okay with it), I think the best choice would be to talk about it and let her choose. While she doesn't have to be locked into using diapers forever, denying it from her will likely cause a bunch of problems from now until it resurfaces many years later, at least in my experiences.