Ok, that does clarify a few things.
I did replace the chain, as the old one had cracked in half, and turned itself into a rust sculpture.
I worried that there would be an uneven wear issue, however the teeth of all the cogs looked practically new. The chain fit very nicely into all the cogs, I checked them all.
I will look into the flush/lube options thanks. I will also tell the guy I built it for that he should likely just replace it.