Other than growing a few more hands between now and your trip, I'm not sure what you can do.
Sorry, couldn't resist.
I've seen your other posts and I can tell you are really having a hard time puzzling things out. What you are doing is very nice, though, so keep trying. We'll try to help you out when we can think of something.
1) Cost: I know those ecvs are expensive to rent. If it's any consolation, my DD's power wheelchair cost more to buy than the first 3 cars my DH and I bought (the combined cost of all 3).
I see you are staying off site. The onsite resorts have some wheelchairs to lend free of charge, first come first served. You resort might too. That would be worth a call to them because it would save rental costs for you.
Another alternative might be to see if your area has a lending closet for things like wheelchairs that you could bring from home. Try calling United Way in your area. They usually have listings for services provided in the area (the UW "help" line is often called
First Call for Help)
If you are flying, the airline has to transport a wheelchair for free.
2) Growing extra hands: I would look at alternatives to a stroller. Eons ago, when my kids were little, we used a baby carrier for our youngest DD. Strollers can't go into the lines or into buildings, so having the baby carrier was a big help. For you, it might be vital since you can't carry a baby and push 2 wheelchairs with only 4 hands.
You might want to post on the Family Board to ask about alternatives to strollers. I've seen some people who swear by slings and backpack carriers, even for babies older than 14 months (much older).
If your stroller is small, you may still be able to carry it with you so you don't have to push it all the time. People fold up walkers and bungee cord them to the back of ecvs and wheelchairs, so you could probably rig something up that way with a folded umbrella stroller.
If you don't have a backpack or sling, maybe you can find a friend or someone who will lend you one. You might be able to find one at a bargain rate at Goodwill or a resale shop since kids outgrow them.
Good luck. You sound like you are trying to look at all the angles, so I'm sure things will work out.