I had never had one up until a month or two ago. I didn't read the whole thread but here are my answers.
1. My overflow is right at the top, maybe an inch below the surface. 2 x 1" ish pipes with screen covers.
2. There is not right/wrong amount really. Just so it's enough to never go dry and so it's not so much that it's about to overflow really. At least that is how it's been for me. I keep anywhere from 50-100+ in my 200g sump.
3. I actually do most of my water changes out of the sump now, I don't like to bother the fish if I don't have to. I can't even dent it, so I have no worries of anything running dry. With a small sump/setup you may have to keep a close eye. I start mine and go watch TV or something, it takes a while.
4. It depends what kind of food, I feed all sorts of stuff and I have never had an issue. Most foods I feed sink though except krill but it never gets stuck to the overflows from what I have seen.