We use what is called a portable travel wheelchair that folds up and fits in a lot of cars. If you are flying a wheelchair is not counted against your luggage. Anyway, you can get one for $100 to $150 depending on the model, not counting tax or shipping. You will never have to rent. The downside is there will be a stigma owning your own, but in the long run it's a good answer.
The rollator is a good aid for balance issues. The downside is you have to walk with it. Yes, you can rest, but then you are not moving.
I know you feel DH won't use an
ECV, but you can really last a lot longer at the parks with a
scooter. By lasting longer you get better value out of your park tickets. Most importantly, you don't have to hurt and you don't have to push; so, you will last longer. The average walking at Disney World is 6 miles a day. Of course, you aren't walking constantly. You can take scooters through many of the queues. There will be a place to park them in the ride. You can also park it outside of an attraction and walk the ride. Infact, you can park it in a section of the park, as long as, you don't block anything important like a parade and then go back to it when you are done with an area.