Sue already addressed the changes in policy for aids, so I'll not repeat that.
Please do NOT go down the route of "I'll just rent her a wheelchair at WDW". These are only adult sized chairs. Not just that, but rather big adult sized. And as rental aids tend to have to be in this situation very sturdy and easy to clean but that does result in very little comfort or support. For a child to be put in an adult chair it is very uncomfy. For a child with eds that already has joint pain? Asking for a sure bet to make things worse. She will have no support where she needs be and will be forced into positions that can increase pain, cause subluxations or full luxations. She will really need an aid that fits HER size. Any aid that is too big or too little will result in lack of support for her joints (and thus best situation to rest and not increase pain), increase of pain and depending on how her eds is and the specific aid can even cause bigger problems like luxations.
This can sound like a big pain in the butt, right? Not so much so, but it would require you to tackle it by getting her an aid at home and bringing it along. Start long enough before the trip so you can try out if the aid properly fits her body. If she finds it lets her rest or would still require work to sit in it. You can try out some cushions to see if that tackles some smaller problems. Not just that, but airports can be loooooooooooooong if you are flying. So at the airport you'ld rather have it there and not need it than vice versa. Simply gate check it, free of charge.
Lines; ALWAYS bring the aid with you in the line. Again; rather have it with you and not need it than vice versa. If she wants to, she can stand up there is no rules about must be sitting down at all times. A GAC is not there to shorten lines or deal with stamina like being able to stand for only 10 minutes. Mobility aids are the appointed help for that. Even if a wait seems short, take it! If a ride breaks down or takes longer wait than expected, that 10 minutes the line was mentioned to be can turn into 30 minutes or longer. Taking a mobility aid into line does not mean she is glued to it. Where she can and fitting, she can simply get up and get onto a ride when it is her turn. CM's working the ride will deal with her aid and have it waiting for her when you guys get off again. If transferring is too much or for instance a ride seat wouldn't be doable for her; on certain rides she can stay on those mobility aids that have offical tie down hooks. So it might be worth keeping that in mind when aid-looking.
Also keep her spirit in mind with that aid-searching. Strollers are well..... strollers. Enough 8 year olds aren't always that comfy with something "babyish". For them it can be a lot easier to accept using an aid that has a more obvious aid look like a pediatric wheelchair than a stroller. Obviously SHE is the most important, but the difference in look can also influence how others look at the situation. Not a biggie at WDW imho, but can make quite a difference in some other settings.
It might or might not be of help for this trip depending on scheduling, but have you considered getting her an OT evaluation? Given the situation it can be good odds that she'll need a mobility aid for longterm for longer distances. If so, it can be wise looking into coverage to get her an aid that is fully set up to HER needs instead of general use, focussed on HER abilities and problem points. In any situation a custom aid can make a huge difference compared to a general aid, but esp. in eds when certain stuff pop up. Might be very worth it in certain situations and if so an aid simply being there, available for use if need be, can make it much easier on her and you to use it when needed and reduce her overdoing it or (unconciously) not always doing stuff because it's too much for her mobility wise.
In general? I'm one of those with veds, happen to be on the worser end of the scale. Pacing is your keyword. She might likely not be a good teller of when is enough. Difficult enough as an adult, let alone as a child. Let alone in a place like WDW. So you might find yourself needing to help her a bit by that by getting rest moments before she is burnt up. She'll be able to do more when resting a bit sooner than she will be able to do on a day if she goes on until she's fully burn out. Recouping from that takes longer and is more difficult than recouping from a battery that is only half empty instead of having even run out of your emergency battery. It depends on personal problem points, but for many swimming can relax the muscles and thus joint pain also a lot. Many kids love swimming, so you might try to schedule some park, rest, swim, park for instance.
For quick rests; go down to first aid or baby care center in each park. She can use a bed there for a quick rest instead of needing to go back to the hotel. You can also use it for any other medical stuff that you might need to do.
The more you know her triggers the better! For instance if unexpected fast spinning motions result in a luxation of a shoulder, you can plan accordingly to that knowledge. For instance not do multiple rides that have that motion on the same day or if it is a very painfull/risky trigger for her; skip it all together. Same with seating. She'll be able to go longer/have less pain when sitting in a supported way instead of seating that results in her "working" to sit or for instance leave her dangling her legs etc. Something as simple as a pillow can make a seating much more helpfull for her during for instance a dinner. Or if having good comfy support in her mobility aid; staying in that for a show instead of sitting on that hard wooden bench without back support that hurts her or uses up too much of her energy to sit on it.
Lines; as said take the aid along. Also take along portable "entertainment" if you know that helps keep her relaxt instead of jumping all over the place in excitement of getting to ride her favorite ride. Jumping is fun, but will take energy she now can no longer use for some other disney stuff. Whereas some entertainment she likes can help her reload physically a bit and she'll be doing something she likes. For instance a favorite game, puzzle she can do with pen, etc. Or do line-bingo where you give her a list of for instance 10 items/colours/caracters you know will be seen and she has to look out for.
If she has difficulty relaxing during sitting even when properly supported (which will result in too high muscle tension, which will cause more pain etc.); it can help to look into known aids for those with sensory integration issues. For instance a weighed down blanket on a lap during sitting can help someone with eds to relax their muscles to more "normal" muscle tension (and thus less pain!!) because now the needed support for joints comes a bit from that weighed down blankie. Can be a search to get the weight just right (too light and too heavy will cause problems, and the difference can be as little as an ounce), but it is so worth it! Not just a nice help to relax in the room or while she sits down in the aid but in daily life also. When buying these aids, they are very expensive many times. Many can be made yourself, and weighed down blankets are very easy to do. Much cheaper and she can pick out a fabric she loves. To see if it is of help for her; many times one can try out aids for a period of time through an OT, PT or by asking the manufacturer/dealer.
And best of all? For those of us where outside pressure helps to ease tension and pain, that same result is seen in a just right places and right of "force" big hug!

What better way of reducing pain?

Obviously need to watch out a bit for a wrong placed hug that can sublux a joint but you'll both soon learn what works best for her. And she'll have the best ever reason for hugging her favorite Princess or other caracter that bit longer.
And totally not important, but might be a nugget that might be of help in the future; 2 types of eds in one individual does not happen. Or let me rephrase that; never say never, but it would result in eventual medical publications and being the first worldwide. Often it does happen that folks get a bit misdiagnosed, due to so much lack of knowledge about eds. Many, if not most, of us with eds will have an overlap of symptoms. Many in the medical field look at those symptoms and then go; thus it must be each type they have symptoms of. Genetic studies have shown no such thing, but actually having one type with an overlap of some symptoms that can also be seen in other types. Misdiagnosing of those with heds to be labeled as also having veds is a common one in that, as in vice versa someone with veds to be mistakenly labeled with also heds. Not all that hugely important for now, but it can be a nugget in fishing out the more knowledgable docs versus trying very hard but not that much into it. Also will become important later on in case of any wishes of becoming a mom she might have and might be important for your family given the hereditary info.
Given her veds; did you know about the research done on cardiovascular complications in veds and how one specific medication can decrease the risk significantly? Obviously not something with a child one would ever go "oh just pop a pill", but if she were already on certain meds it can be worthwhile looking into if it might be an option in her treatment. Quite a few people who were already on certain medications for cardiac symptoms could easily switch from their current med to this studied med without real risks or other group of possible side-effects, but does come with that benefit of decreasing the cardiovascular complications of veds. It's still pretty "new" and very little known with many cardiologists, so reckoned I'ld just mention it it case you don't know yet and want to know in case you ever need to consider certain meds. If you want to know more, feel free to PM and I'll give you the name and link of the study.