I'm not greenliner, but I recently from high school in the chicago suburbs and ran with York a few times, and was friends with a few guys on the team, so I could answer your questions pretty well.
1.) Yes, they work very hard, but some of those kids are very talented. The Dettman twins were very good middle school runners and were going to go to willowbrook high school but moved into the York district right before high school, so obviously York might once in a while get move ins like that. However there are guys that simply work hard and get very good as well. Mark Fruin, who ran 9:16 and 4:16 this year, was like a 16:00 3 miler last year or something like that, I know he wasn't on varsity or anything as a junior, and he ended up taking 4th in the 3200 at state the next year.
2.) I'm pretty sure anyone who runs 100+ miles a week runs the upcoming season.
3.) Yes York runs indoors, although they do not run the unofficial indoor state meet at Uof Illinois. The indoor track when I was in high school was called the dungeon it was so crappy, but I heard they just got a new one.
4.) I think it depends on the year and team. With a team like this past years, I'm pretty sure there were kids that ran 1,000 miles and weren't on top 10
5.) Sorry, I have no idea when their summer miles start/stop
6.) I don't know how good they have traditionally been, but the girls team has been very good when I was in high school (graduated in 03). They won state in 2000 and placed high every other year, usually have a few all staters in cross and/or track. Maria Cicero won state in 2000 and was a footlocker finalist.