As far as a GAC. I didn't request one and we didn't need one. We went in Aug and it was near 100 degrees every day. HOWEVER since she was six years old I still felt comfortable using a stroller and we rented one and used it. Not all the time, but enough. If we were just walking from one attraction to another nearby, say in the Magic Kingdom we didn't use it. But if we were crossing the park we definitely used it. We used it a lot more at Epcot as it's so big and the attractions are further apart. I think that helped a lot at avoiding the lows. We used FastPass whenever possible. Even though it was crazy busy I think our longest line was maybe an hour when it was later in the evening and no FastPasses were available for Big Thunder Mountain. But I believe most of that line was in the shade. I really can't remember any rides that we waited out in the full sun for. Maybe Dumbo and the Aladdin ride? But for Dumbo I knew it was a full sun line that can be insanely long so we ran for it as soon as the park opened to avoid that wait and only waited for one ride cycle. And even the Aladdin ride line wasn't too long from what I can recall.
I also have absolutely no hesitation at testing and treating wherever we may be. In line, on the rides (not the wild ones of course) etc. So I can't see needing to get out of line except for severe lows/bathrooms. And that wasn't an issue for us. I tested a TON and at that time she rarely had highs so no bathroom issues.
For us then it wasn't necessary. Now we can't use the stroller so I won't say never, but unlikely with lots of planning etc. I guess if she was high and needed to get out to use the bathroom I would try and speak to the line attendant to see if they could let us back in etc.