You have the same issue in classical mechanics. When you drop a bouncy ball onto a hard floor its momentum reverses. So, what happens to conservation of momentum?2) You said "At some point"... And what is that point? Where is it? It is not "proven" threshold? OK
The theory says that the Earth's momentum changes by the same amount in the opposite direction. But, that is untestable as it's impossible to isolate the Earth from other impacts or the effects of other gravitaional bodies in the solar system; not to mention actually identifying and monitoring the motion of the Earth's centre of mass.
Instead, we test conservation of momentum where it can be tested and infer that the law holds even where it is impossible to verify directly.