I'll answer a few
3) see 4
4) this is the way they should be wired. You'll need 2 resistors per motor.
6) Ebay -
http://cgi.ebay.com/ws/eBayISAPI.dll...Name=WDVW&rd=1
7) It might be OK, but will probably run hotter. You'll need even bigger resistors, though, right?
8) low speed torque will remain the same. useable high speed rpm will increase, proportional to voltage. 50% more voltage should get you about %50 more speed.
9) They turn, but probably not much more.