So lets say best case scenario the i-601a is approved and I depart to mexico for my interview, after the interview is over i will be able to come back and wait here in the us?

If your waiver is approved and you go to your visa interview and everything goes well at the interview and you are approved, you will receive your immigrant visa package, which means you have been approved for US permanent residency. You will return to the U.S. as a permanent resident, and your card will arrive in the mail a few weeks after entering the U.S.

OK I understand now, Thank you :D