Immigration Site

Hi, Everyone.
I recently got my Conditional Green Card for 2 years. This is family based GC. I wanted to change my last name to married name on the card. I was told to file for I-90. Because my GC wont be expired till 1 year 11 months so I counldnt file the I -751, yet. Anyway, I have been reading the posts which are about I-90. Eveyone looks like frustrating for waiting again. I can understand it well.
If anyone can help my quesitons, I appreciate that.

1) Whats the difference between efile and mailing to mail box in LA?
2) Which is better or faster?
3) I undersatand if you chose efiling, you will take all supporting document when you do your fingerprint. How about mailing to LA office? Necessary to send supporting doc along with application and fees?

4) There is a section to complete saying " If you entered to the U.S. with immigrant visa, complete following: 1) Destination in U.S at time of admission, 2) Port of Entry where admitted to U.S." ----I didnt come to the U.S. with immigrant visa. I came with F-1 Student Visa. What should I do with this section? Can I leave blank? or Should I put "N/A"?

5)There is also section about city of birth. Should I include country, too?

6) About approximately, how long for waiting? Does it matter where you are? or not.

Thank you.