Hi all, here are answers to the last few posts:
@speedbump: the water is from the city, we have a whole-house filter on the incoming water line, nothing being caught there. There's also a filter on the incoming tankless water pipe, nothing caught there either.
@djmayhill: the basement is unfinished and it looks like copper throughout - the plumbers who came checked the same thing - no telling if the pipes change once they go upstairs behind the walls there, but I guess that would be really unlikely?
@speedbump: we had the water tested and the pH is 6.8.
Luckily we are just renting this property, so we'll try getting out of here. I'm not sure what I would do at this point if I owned the house - cut out some sections of pipe, open them up and see if there's damage in there like in that video I posted?