OP, you describe two distinct needs.
For the "hard to stand for extended periods" issue, Disney generally considers that to be a stamina/mobility issue, and their solution is for the guest to use an ECV, wheelchair or similar mobility aid. As others have noted, if you "need a seat" you pretty much have to bring it with you.
Many people who rent
ECVs or wheelchairs for Disney trips do so ONLY for Disney trips and do NOT need them at home. But the distances and/or amounts of standing/being on one's feet involved with a Disney trip can mean they DO need such a device in order to have a better experience at Disney. I am one of those people. Don't need it at home, but to do WDW, even for one day, I need an ECV. So I rent one from an off-site vendor and consider it a cost of my trip -- but it means I am in less pain, can do more, and have a much more pleasant trip for myself and my family than if I did not rent one.
For the "need to pee NOW" issue, this may or may not be something Disney would agree could be accommodated with a
DAS, since ultimately the issue affects your ability to spend the wait time IN the line environment. Nowbody here can tell you if they will, though, it is something you would need to discuss with guest services and they will decide. Keep in mind, their concern is not your diagnosis or the specifics of your condition(s), but rather what your NEEDS are as they pertain to waiting in line or why being able to wait outside the line would be better. Since they may say "no", you also need to have a backup plan/strategy for how you will navigate the parks and lines. Even if they give you a DAS, you may find that backup strategy works much of the time and you need minimal use of the DAS itself.
SW