Hi Scooterbritches.
Just a few quick thoughts for you, after reading all 4 pages...
First, if you are going to use a wheelchair at WDW, please remember to bring along a set of bicycle gloves to help protect your hands (or those of the person who pushes you). Regardless of whether you self-propel, or push with the handlebars, your hands can blister and become sore and tired much quicker than you might image at someplace like WDW where it is easy to go 10 miles in a single day.
If you are not used to self-propelling in a wheelchair, it can be (and will be) very tiring, very quickly. WDW has far more hills and ramps (even gentle inclines seem giant when your arms are tired).
If you decide to use a wheelchair, another tip is to pad out the seat with beach towels, or bath towels; not only does this help prevent that uncomfortable sweaty feeling from sitting in a plastic sling, but it also helps keep you comfortable as the day stretches on.
Remember when using any wheeled device to cross the tracks that embedded in Main Street (in the Magic Kingdom) at an angle to prevent the wheels from getting stuck in the tracks. Approach all ramps and hills slowly, regardless of whether traveling uphill or downhill.
Regarding Rollators, some can act as both a walker with a built in seat AND a transfer chair, making them potentially more useful to you. The dual-purpose Rollator will have wheelchair-like handlebars for pushing a seated rider on the "front" that often swing inward and lock when not in use.
I recommend checking with local service agencies, charitable groups and religious organizations (especially the Masons) to see if they have any equipment that you can borrow for the trip. Remember that you may find that airport lines are troublesome as well; We waited for 45 minutes on the way home at MCO just last month (on an early January Wednesday afternoon) to get through TSA screening; the lines are MCO are just stupidly long and slow sometimes. If you are unable to borrow the equipment for any reason, in addition to local thrift stores, check CraigsList to see if you can find low-cost gently used equipment. Needless to say, any used equipment should be cleaned thoroughly prior to your use.
Finally, an option you may want to consider, and that I have not seen mentioned, is a "transfer chair". These are typically quite lightweight, and can fold down to a relatively compact size. They are meant to be pushed by someone - the person sitting in the chair is a rider, and does not self-propel due to the fact that all four wheels are the same (smaller) size. These can often be purchased new for about $100, making them a very economical solution as well. Because the transfer chair is so lightweight, it makes it easy to push when you need to walk, and because it folds down so nicely, it makes it very compact for travel and storage.
As others have mentioned here, any personal mobility device you take on the plane with you will fly at no charge. If you take a wheelchair or Rollator, you may want to take along a couple of Velcro straps to keep it folded in the cargo hold of the plane; this helps prevent damage to your device, as well as to the other devices and strollers that may be flying along in that hold.
I understand your distress; I was 51 the first time I had to rent an ECV at WDW. It nearly killed me to do it, but it made our day so much easier - suddenly, I could move at the same pace as everyone else, instead of holding up my family while I struggled along bench-to-bench. I had to accept that my ECV is a tool that I use to accomplish a task, and nothing more. Just like I need a calculator to do math (because I *suck* at math LOL) I need an ECV to cover longer distances because my legs suck at walking. My particular needs are met by using an ECV, but you alone know what device or solution makes the most sense for you. Whatever you decide to do, I hope you have a wonderful time with your daughter!