How likely would it be that terrestrial immune systems evolved to fight Venusian microbes?

It could be as simple as a microbe that eats an elementary molecule in our bodies that is so foreign our immune system can't spot it. No need to target us directly.

How likely is it that a virus evolved for living in civet cats just happened to evolve to also be pathogenic in bats and humans?

Quite likely; we’re pretty closely related to those, and multiple species viruses show up all the time. It would be more surprising, though, to find a virus which was pathogenic to, say, both us and archaea, and any Venus life is probably more different to us than archaea are.

A bacteria analogue is possibly more of a risk; those can be less fussy.

We get infected by bird viruses all the time, and they're not even mammals. A virus that evolved in mammals infecting different types of mammals is very likely.

Not likely, except if it were somehow, coerced to evolve, like, in a lab.