Yep! It's slightly informal, but very common.

You can shorten "good afternoon" to "afternoon." I have heard it before. Actually I've heard it being used by people from England. I haven't really heard it being used by Americans. I wouldn't really use it in a formal setting.

Yes that is very common to hear in the UK, and by the looks of it in Australia too.

It is okay, however informal to shorten "Good Morning" to "Morning" and "Good Evening" to "Evening". It is not as common to shorten "Good Afternoon".

Yes :)