Be careful stepping outside your field. I used to work at the NASA Astrobiology and Lunar Science institutes. There is zero chance there is any biology going on or under the surface of the moon.
Is there anything close to that pile of evidence for life (whatever that means, maybe not necessarily carbon-based) on moon or lack thereof? Now do Mars? Personally I don’t think there’s anything on moon, but the evidence isn’t much. In the 60s it might as well have been nonexistent.
Mars is a different story.
Impacts would destroy any conceivable microbes in the immediate vicinity of the impact. But unless an impact was strong enough to sterilize the entire moon all at once, microbes outside the kill radius could subsequently reclaim the impact site. Like plants reclaiming land after a lava flow destroys everything previously living in that spot.
I agree with your other arguments, and believe the moon is sterile.
And visibly alter it. Even microbes consume matter from the environment, and excrete metabolic byproducts. It would be detectable at a chemical level; with enough time, even visually.