I wanted more explanation of how the Martianisation occured if all the Martians had died off.

Yes, if got a bit vague - they'd polluted the air, but that didn't really explain the weed and the rocks.

If they thought they were sneaking in a searing, ultra-modern critique of colonialism, they weren't.

I agree that they hadn't thought through the post-apocalyptic timeline, although the fact I found it easier to follow in the final episode suggests to me that if it had been written differently, it might have worked.