As far as routines go, (both my adult daughter and I have been diagnosed - she was diagnosed at age 11!) just *sticking* to one, especially when we travel, is key. That means defined meal times, scheduled meds and above all else, getting plenty of rest - going to be on time, every night. It's not always easy, as I'm sure you are aware; at Disney World especially, its easy to lose track of time
What has helped me the most has been aquatic (pool, or water) therapy. I take regular physical therapy sessions, but all of my workout is in the pool. The pool at the PT place is kept heated *very* warm; it's typically between 97 and 98 degrees! When you first enter, it can feel almost too warm, but every time, by the end of my session, the water feels almost cool - no kidding. My therapist has a series of exercises that I do that are designed to help my arthritis, my fibro and some of my other neurological and physical issues; we even do balance work in the pool.
It's not for everyone, and not every PT place that has a pool has one that is heated that warm, and not every PT place that has a pool has someone who is really skilled at aquatic therapy. But, it has made a HUGE difference for me, and has meant that I can still walk and stand unassisted, and that I am no longer losing ground, and have made (very slow, but measurable) progress.
Fibro is a cruel and horrible disease, and I would literally take every bit of it in the universe if it meant that no one else ever had to suffer with it again.
When I am at WDW, we stay at the Resorts on site, and I get into that hot tub every night, (and sometimes again in the morning, weather permitting) to help my muscles stay loose - and I do a modified set of excerises there, just as if I was at home!
Your husband might take some of his cues from you - if you are reluctant to use an ECV, and talk about how you hate it, and how you don't want to use it, he may be (either consciously or unconsciously) adopting your attitude. Try talking to him about the "tool theory" (it removes a lot of the emotions attached to the discussion) and then I hope you both will know this:
You - either of you, or both of you - on a scooter at WDW is a total non-event. I promise. There are so many people who are on their phones, bragging on Facebook about their "perfect" vacation, or trying to find the next "Instagram wall" or taking selfies, or looking for the (fill in the blank with the latest and greatest "must have" millennial pink/unicorn/candy-colored food/drink/spirit jersey) item that
no one will even notice you, on a scooter. Seriously.
Oh, and don't worry about seeing anyone you know at Disney World - because you have a better chance of getting hit by lightning *and* winning the lottery on the same day than you do of bumping into someone from home, unless they traveled with you.
And that whole "stigma" thing? Who cares? Certainly no one at Disney World. Just take a moment and look around - everyone there is so self-absorbed in creating their own perfect vacation that they really don't notice anyone else. So unless you (literally) run over someone, you will just be background noise to 99% of the folks there - just as you are when you are walking. No difference.
Use that scooter as a tool to do a job (the job is to carry you when you need an assist) and go on. Don't get hung up on the fact that you have to use a scooter. Instead, remember that if you *do* use a scooter, you will arrive back at your hotel room every night in less pain, and less exhausted, and more able to relax and enjoy your evening. Think about how great it will be at the end of a long day at MK to have a place to sit while you wait for the bus! And finally, know that Disney World is a use case all of it's own; no where else would you be expected to walk between 3 & 10 miles per day, with little to no shade for much of it, and little to no seating, benches or rest areas. It's not the "real world" that you live in at home - it's extreme in every sense of the word.
Don't feel guilty if he refuses to try using a scooter. Go ahead, and rent one for yourself, and just keep the phone number for your rental vendor handy in your smartphone for when he finally says he kinda wants to rent one too.

Because if he sees that it is a *positive* thing for you, that may be all that he needs to make the decision. And if he decides to be stubborn, and refuses to use the best possible tool available for a completely non-standard situation? Well, then you may have to just let him go on, and limp his way through. But don't feel guilty for using all of the resources at your disposal to have the best possible vacation!
You can always rent the scooter(s) for just one trip, and try them. If you don't like it, or they don't help, then never rent them again...