You keep using that word... I do not think it means what you think it means..
Probably, but it would likely be "Så utrolig da!" if you wanted to be sarcastic.
Da usually means then, but it is in frequent use as a filler word... not meaning much. It would be strange to translate it to then in this context anyhow. Or could you say things like "how about it then?" where then doesn't really carry a meaning?
Yeah when I heared it while I was watching NRK I thought it was "det" but it wouldn't make sense, tusen takk.