1. They don't go insane, they just get (increasingly bigger) sad thoughts the longer they are on duty. In theory if the dwarf is happy enough from other things (somewhat hard if they are not sleeping in royal bedrooms and eating in the giant dining hall) they could stay on patrol forever and there would be no problem. As for how long it takes, I don't know for sure but it's quite a long time, at least a month and probably more like 3 to get the first sad thought.
2. Yes, if you have them carry food and drink they will eat/drink that first then start going back to the fort when they get hungry or thirsty. Dwarves don't currently have the discipline to stand there till they starve to death even if ordered to do so.
3. only if you set them to use crossbows, then they will use up all your bone bolts. However, sparring is a good thing and once they get legendary shield use and legendary weapon skill the chances of them getting a wound from it is so small it's not worth considering and so you want them sparring as much as possible.
4. Unfortunately, no - any dwarf who does not own a room will prefer the barracks beds/floor to sleep in. Make rooms for all your dwarves and it won't be a problem, or if you don't want to make them rooms just add lots of beds to the barracks - sparring dwarves won't bother sleeping dwarves at all.