Having been the "kid" in a similar situation, it sounds to me like your DH is having trouble coming to terms with the fact that your daughter has mobility limitations at the moment. I went through a lot of that with my father has I became less and less mobile. I'm 28 now and I bought my first wheelchair at 20. My father hated the fact that I had it. Anywhere we went that I thought I'd need it, he'd ask "do you really need the wheelchair?". I'm not a big fan of conflict so I spent a lot of time limping through museums or ending up not seeing anything because all I could do was sit. Since you're willing to stand up to your DH on this (my mother wasn't comfortable with the wheelchair either so I had no back-up), hold firm. These days my father has learned, but it took years and hearing it from medical professionals and a bit of family counseling. When I finally got a diagnosis (after 10+ years of trying), it also helped since he really couldn't try to will the problems away. I certainly hope your daughter doesn't have to wait as long as I did for answers, but these are all things that may be subconsciously going through your DH's head.
Does your daughter have any problems with sitting with her legs down (like in a normal chair) or is it just walking? If she's fine sitting, I'd suggest the
ECV. She would definitely struggle self-propelling a rental wheelchair and the last thing she's going to want on a surprise trip is to be dependent on someone else to push her around all the time. I'd also urge you to let her choose when to use the ECV and when to walk (you can park an ECV in a section and walk around if you'd like). There are lines that are 1/4 mile long and there's no way to cut that distance down even if there's no wait time. If she can only stand 30 minutes before she starts having problems, she may be better off planning to bring the ECV into more lines than you're thinking because that means that those 30 minutes have to include the time walking from the scooter to the line, the time in line (even if it says 20 minutes, what's the plan if there's a delay?), and the time back to the ECV. As someone with chronic pain, I can tell you that the best way to control pain is to do your best to prevent it.
I look young for my age and have been to WDW plenty of times in a wheelchair (both manual and powerchair) and I've never gotten any looks or anything. If they're there, I certainly don't remember them. So if that's something your DH worries about, he shouldn't. There are plenty of younger people using
ECVs or wheelchairs at WDW where there are non-obvious problems.