3. Biologists are easily able to determine if they are new kingdoms are not. They're also able to estimate how long ago divergence from a common root happened.
There are many, many examples of parallel evolution in terran biology, but none of them are confused with each other. It's absurdly unlikely that a terran modern amoeba will be confused with a Martian amoeba.
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.
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.
To answer part 3: We're still discovering new kingdoms of life on Earth (though it's unlikely we'll discover new domains). If localized panspermia exists within our solar system (from meteor impacts or the like) it's possible martian life and terrestrial life are related enough for the martian life to fit within the already existing family tree of terrestrial life. https://astronomy.com/news/2021/05/did-life-on-earth-come-fr...