Thanks a lot for explanation and reassurance. I will check the chain stretch tonight. I believe I can still measure it when the chain is dismounted if I put some tension on it. Cassette looks like new when clean, don't see any wear on it whatever. I think the shifting under load that scuffed the chainring was probably done by the original owner who had it for the first three years.
But last question remains - 172.5 vs 170 mm crank arms. I tried to search if there is thread on crank arms length but didn't find anything, I wonder if I should make new thread on it. I imagine it would have already been discussed and flogged to death (with everybody getting tired of it like the greasing thread) but maybe that should be in other forum sections?