-It will be mailed to you directly, not to the lawyer.
2) How long it takes?
-I don't know, case by case.
3) Is it in a regular mail or the mail that needs to be signed?
-Regular mail (not need to sign it).
4) what do i need to do when i come back to the US?
Must i notify CBP that my green card is approved while i was in CHINA?
-Yes.
5) Is my AP still valid? Can i enter the US with my AP?
-Yes as long as you come back before the AP expires.
Thansks so mucb for your guidance, Teacher8