iRig setup question(s)

I have a iRig STOMP, see here, in the post.

My questions really are the usage of multiple audio outs happening at the same time, in a way.

First question. I've got my guitar plugged in. It's then plugged into my Mac. I've got my headphones plugged in. If I have a song playing (Spotify/iTunes/whatever) do I hear that song through the headphones, plugged into the box, at the same time as hearing guitar (through garageband or main stage)?

Second question. I've got my guitar plugged in. It's then plugged into my Mac. I've got my headphones plugged in. And I've got it plugged into an amp. If I have a song playing, what happens? If I don't have a song playing what happens?

Third question. The iRig original works on Mac, so I have just presumed the the STOMP works on Mac as well. Am I right?

Ok! It's arrived, to those of you who wanna know.

First answer. Yes, you hear any audio playing (Spotify/iTunes) through the headphones.

Second answer. Dunno, don't have an amp.

Third answer. If you are using a 3-way headphone cord from the box to the Mac then yeah. Basically, use the one they give you.

I would highly recommend using something like Main Stage and not GarageBand.