1) The 6 year programs also grant you an MD degree (extra 2 years in medical school). After a 4 year program you can go into private practice.
2) An externship is a 1-4 week rotation at a OMFS program during the 3rd and 4th year of dental school.
3) Everything extra or make your application unique will be helpful. As a first year your best bet is to "hang out" in the OMFS department and get to know people there.
4) Best bet is to call the program up and ask but I think just do well in D1 for now.
5) GPR is usually 1 year. You can apply during or after your GPR.
6) You apply during D4 and before part II.
Hope I am right about these info...