Very common in the US and Canada but probably not outside (updated my comment).

Common in New Zealand.

Not in the UK. I'd be very confused by that usage. "Demo" means "demonstration" or "demonstrate". Maybe builders might use it that way but I've not ever heard it.

My friends in the trades have used the word fairly regularly. I'd say yes.

Native-born bi-coastal boomer American here. Never heard this usage until my house was remodeled and the general contractor used it. I had to stop him and ask what he meant. (I didn’t feel too stupid since the vowels and syllable stress are different.)