It becomes much more difficult to determine whether the planet contains indigenous life once you've introduced life from Earth.
b) we should have found something by now. There clearly isn't a whole lot happening on the surface of mars at any rate
b) we’re still guessing at bio-signatures; all we are confident of is there isn’t “a lot of stuff like us”, but that’s a compound claim and we can’t rule out “a small quantity of stuff like us” or “as much life as a desert but very different to us”.