Regarding ATC Watcher's earlier point, yes I think we are talking about the same thing. Should have expressed it like this before, but the main exception for sense reversal of an RA is when both aircraft are in level flight. A very mystifying exception.

I witnessed this in a sim a couple of years ago (it was the latest TCAS version). It was impossible for the instructor to "set up" a TCAS encounter with another a/c in level flight. Seems to me not a very clever assumption, that one of the a/c will always be climbing or descending.

The TCAS warns you of a "guaranteed" collision, imminently.

I don't think it would be accurate to say guaranteed collision. High risk I think would be more accurate.