K, hopefully your eds is not such a ***** as mine.

But still; take it very serious. Overdoing it can be so damaging. I though I understood and took it to hart during my first years (also still walking around small amounts) of having problems due to the eds, but turns out I had no clue and permanently damaged myself with my "I don't want to be a pain in the butt, don't want to need help, yadieyadieda".
Now I'm all for making an informed choice to overdue it and take the consequences of that choice. That's what I just did last week, so I'm no stranger to that.

I just want to make sure you take it serious and not to damage yourself too much.
From the Epcot entrance to Soarin' is a relatively -for wdw- short distance, but I think it would be asking for it for you to walk up the hill to get into the Land Building. Also I found it to be more crowded around Innoventions which could mean you needing more time and energy. I ran into a broken elevator at the Land building last thuesday. That again can be a risk as you need to be on the first floor for Soarin' and get into the Land on the second floor, with no seating option from where you can see if the elevator works again. They ended up taking me and other wheelers in and out via the backdoor.
The lines for Soarin' are mostly uphill. That again isn't too great. And if you have to wait they've got some cute games to entertain you, for which you couldn't sit on the floor and play. I say; bring the chair for that uphill going and to play the game, it's fun.
Really girl, use the chair for all it's worth! Doing disney the chair-style is SO relaxing and easygoing. I really had a blast and never run into any real problems because of the chair. (even with the elevator

) I could have transfered out of my chair into more rides than I did. I didn't because I finally learned my lesson the hard way. Better to safe myself that energy -and not getting the added pain- and use that for something else than doing 2 rides with transferring and feeling it.
How to tell them, good question. I never had to tell anyone about the manual. I ended up in hospital due to so much pain and not being able to get out of bed. That pretty much made it obvious enough.

Now for the powerchair, that's a different thing! I didn't feel good about it myself, so let alone telling others about it and having to deal with it becoming more real because of that and knowing it would cause them emotional pain for me.
I ended up just applying for one and talking to them about the applying. Some got it that way, others were still like "but that's for some moments and not whole day, right?". In my case people didn't really really found it sinking in untill they just have me around in the chair. That makes it real and then they can deal with it like "ok, she needs it, it's for the better". I think all you can do is talk to them and try to make them understand. Good info on eds is the most important I guess as this will stress why you shouldn't overdue it, can't predict when you'll be overdoing it untill that specific moment because your body has a mind of it's own and why you really need the specific requirements of your own chair and not just some wheels to get you round and "oh well the tires and cushion is more comfortable". Maybe they'll just have to see for themselves how much of a difference it'll be for you. Probably you also just have to experience wdw this way to really feel how much it can help you. I knew I needed the powerchair. I knew I would be using it at WDW and getting it on rides as much as possible. I knew I would be so much better of then when I still stupidly decided to do an amusementpark in my manual (getting pushed around and only doing small distances myself). I knew the chair not being anyway near 100% would cost me lots of time at the parks, which it did. But I really didn't get how much this (ugly

) chair saved me untill I actually did it.
You know, just think about that seating of the chair! I don't know if it's a cushion or not? (read; easy to take it out and use in restaurants, rides etc!) Just think about having that right at hand for you to sit down on when you need it. How much this would save you. How great it is to sit down on a bench for a show and feel "no, this is hurting me too much, let's get the chair".
I don't know what the exact
ecv options are in the US, so bear with me. I know there are ecv's out there with a non fixed stiring. With those you could let them set the stiring in such a way you wouldn't need to stretch your arms out more than only lifting up your underarms. I know there are different heights of ecv's available as I'm also not a large girl. Actually, I was surprised to see so many very low ecv's at WDW, way more low ones than over here.
Pushing and getting tired. That'll happen to anyone that's not experienced and hitting the parks real time. Allthough you having your own chair is easier than a rental, it can still happen. I know it happened standard with all of my girlfriends -they took turns pushing me) when we would hit Amsterdam and I still had a Quicky2. If that's your mothers biggest concern, talk about how you could solve it. Another option, ecv, you not going all the time?, renting a device that allows you to use power"pushing" on the chair etc. Maybe it's just fear for your dad and our you??