They grow incredibly fast! I don't know how big the new set up will be but figure a minimum of 1 sq ft per chick by 4 weeks and 2 sq ft per by 8 weeks. Ideally you'll have them out and in the coop before they get to that point.
Not in an indoor set up. Of course I don't know what kind of building your grandmother has, if it's a drafty old barn that would be different than a sturdy shed, for example.
With the plate, I think once you have the chicks under it you can kind of tell if it's too high or too low, and adjust...
Any photos of your set up? I don't know what happened to the ones that didn't make it (they could've had a rough ride to the feed store and were weak before they ever landed in your hands).
So you're going to get differing advice on this, but I think the general advice of "95F and then lower...