As far as we know. As of right now, we are reasonably confident that we haven't contaminated it. So if anything resembling biological byproducts turn up we can say with reasonable confidence that we have discovered evidence of historical life on mars. As soon as a human sets foot there that's no longer the case.
That said, personally I'm in favor of manned missions to as many bodies in the solar system as possible.
No idea, but presumably that would be after we've explored a fair bit of it. We've barely scratched the surface.
It seems quite plausible that there could be ground water. If that's the case (and I suppose even if it's not) it wouldn't be at all surprising for the cave systems to contain microbial life.
That isn't even remotely what I said though. Indeed I'm in favor of that in the end. But you're trying to invoke "just one more" when realistically we haven't even started for real yet. So it seems more like you just want to entirely skip the sterile part of exploration where we're careful not to cross contaminate. Which is certainly a valid position (though I disagree with it) but I think you should be up front about that.
That said, personally I'm in favor of manned missions to as many bodies in the solar system as possible.