Hello, Birdeen's Call.
(1) I believe that you are 100% correct: most Americans probably
use like instead of as if.
(2) Since so many people use it, I guess that we may call it
(3) Nevertheless, if you are doing university-level writing, you
might want to use as if.
(4) I think that your sentence in perfect English should be:
Wave your hands in the air as if you just didn't care.
(a) My books tell me that as if is a short way to say:
Wave your hands in the air as [you would wave them] if you
Student or Learner