As reagrds your questions
1.All vintage centre seconds are hand wound. I know of no auto SC
2. I think there are some other calibres beside the 27 SC but am not sure. This calibre is certainly the most common
3.. Not water resistant. Most vintage pieces are not
4. Differences are really aesthetic. Case/ dial differences. As you mentioned SS is extremely rare. Breguet numerals are very rare
5.No hacking
6.See 4. I think you would pay a small fortune for a SS or platinum model. Most are Yg and PG is also rare
I will post my 2 when I get home a ref 1491 sc and ref 2466 SCJulian