2) Localized sure, I wasn't arguing about the entire planet. But introduced life could drive the native life to a local extinction. And if it did so fast enough we would never know the local life had been there.

3) Yes, I know. While this isn't my specialty I work at an organization that does have people that specialize in this. The difficulty would be in definitively concluding whether this is a native divergence that we've just never seen before, or the result of Martian evolution.

2. We've found fossilized remnants of bacteria in rocks, haven't we? There's also ice on Mars. If life existed, we'd find it frozen in the ice.

3. A billion years of evolutionary divergence, with local alien adaptations, is going to be very hard to confuse with anything brought over by a probe.