It's really hard to know about
ECV availability. Many people with disabilities or special needs try to go during the slower times of the year, so a slow time for everyone else might be a busy time for
ECVs.
One other problem I can see with your plan is that if you wait until "my leg started really hurting me later in the day." Once the pain is that bad, it will be very hard to deal with and you will probably be in pain even using the ECV and you might even be out of commission the next day. You would be better off dealing with the pain before it gets to that point.
It might help to think of the amount of energy you have as a checking account. You start each day with a certain amount of energy and things you do during the day affect your 'balance'. Being in pain zaps a lot of energy out of the account quickly. If you end the day in pain, that will effect your ability to sleep and you will start the next day with a lower level in your "account". If that happens, you won't have enough energy to have fun and your whole family may be impacted because you won't be able to do as much.
I agree with the other poster that renting an ECV from off-site would be helpful. Your 2 older kids are old enough to understand that you will need to use an ECV and that they need to 'help' because of your injury. If they don't help, no one will be able to have a good time. Let them know that you want and need their help. Start talking to them now and be specific on how they will help (maybe even a 'reminder sheet' with pictures). Ways to help could include the things you know will be problematic and the solutions might be out of the box ideas.
Some things you will need help with that I can think of are:
- Staying near to mom and dad and out of the way of the ECV.
- Walking
Even though many people don't think of doing WDW without a stroller for 4 and 5 yr olds, it is possible, especially if you are not in Commando mode (which you should not need to be at the end of September). When our kids were little, youngest DD was in her wheelchair and we told our older DD -who had just turned 5- that she would need to walk because we could not push a stroller and a wheelchair. She did walk, with very few complaints. When she was tired, we stopped and rested. We validated her feelings ("I know you are hot and tired. What should we do about it?") and most of the time, she came up with something or stopped complaining. One time she kept complaining about the heat at Epcot and my youngest DD threatened to push her into the lake - that stopped the complaining and has become a family joke.
We have also been to WDW with younger kids since then. A few years ago, we were with my niece and her almost 4 yr old DD. She did very well keeping up the first 3 days. near the end of the 4th day, she said "my legs are all out of walk." We took off the next day and did get a stroller on the next park day. I think if we had not done 4 park days in a row, she would have been OK.
- Sharing the stroller, if you plan to get a double stroller.
The 2 yr old will definately need a stroller and having a double will mean that one of your other children can ride while the other walks. That will help with kids getting tired and also help with keeping track of the kids. Having a way to 'assign' who gets the stroller will help avoid arguements. You might do something like times for trading off - one gets to ride for an hour, then the other. You could also have something specific they get to wear during that time - like maybe an easy to remove special bracelet. The bracelet wearer can ride the stroller; the other child walks. You might also have some special thing that the walker gets - it should be cool - cooler than whatever the 'rider' gets (so it is an incentive to be the walker). The walker might also have a special job to do (can't think of any right now, but having a special job might be incentive to walk).
That will stop arguments about whose 'turn' it is - you have the bracelet - it's your turn.
- Even thought the 2 yr old is pretty small, enlist his/her help too and give him/her a job. That will help with the older kids ("Why do we have to help and he doesn't?"). The 2 year old's main job might be to sit still in the stroller and get into and out of the stoller with minimal help. Probably would be doing that anyway, but it will make him/her feel more important to have a job.
- No whining
Several people I know instituted "whining time". Whining was only allowed during whining time (once an hour). Whining at other times was met with "I realize you are unhappy, but it's not whining time right now." This sounds very simplistic and I'm sure people are thinking "yeah, sure" 
, but I know from personal experience that it does work. 4 and 5 year olds are aware of other specific times, like bedtime, bathtime and mealtime, so having 'whining time' doesn't really seem as strange to them as it does to older people.
You may find though (like happened to my sister), that her children were saying "Mom, it sounds like you are whining and it's not whining time yet."
You can even enlist the 4 and 5 yr old with coming up with solution. Sit down and tell them you will need their help in the parks. Tell them the problem (like who will ride in the stroller) and see what they come up with for ways to deal with it. They might come up with some good ideas that only small children can think of and if they can't think of any ways to help, you can 'lead' them into some of the things you have already thought of. They are more likely to 'go with the program' if they have had some input into it.
