My DD has a kidney issue and needs to drink lots of extra fluid, which results in frequent bathroom breaks, sometimes urgently. We have not found a GAC to be of much help. However, we stop and "try" at each bathroom and definitely go before getting into a ride line. Once you are into a line, it can be challenging but possibly to get out - you'll have to walk past all the people in line behind you. Speak to the CM at the line entrance to explain your situation and see if they can offer you anything to return. Maybe, maybe not. I honestly don't recommend trying to cut back into the line to join your party - many people in line by then may not have been there when you left and may make it difficult for you to get through. So speak to the CM when leaving the line. But best is to use FastPasses and a touring plan to know which park and which part of each park is least crowded, then never pass a restroom without stopping. And, of course, carry a change of clothes for the "just in case" event that hopefully you won't need.
As PP mentioned, some rides themselves can be long so you'll want to know those ahead of time. Check out the disAbilities FAQ sticky at the top of this forum for more info on those. You'll at least need to be aware of these, and possibly skip them if that's too long (line time plus ride time). There is no exiting once a ride has started.
Good luck and enjoy your vacation!