We have someone coming now, and there's a reasonably good chance it'll be at Strata expense and not ours.
Backflow valve sounds great and I'm going to ask, but I suspect that it's not practicable for this particular issue. I think they'd have to dig beneath the concrete foundation to install it, which I don't think is likely. The plumbing here is pretty screwed up, was probably to code 25 years ago, but it isn't now, or so we've been told by multiple plumbers.