For point (2), you can derive accurate counts from sampled data if the sampling rate is captured as metadata on every sampled event. Some tools do support this (I work for Honeycomb, and our sampling proxy + backend work like this, can't speak for others).

The issue is there are still limits to that, though. I can still get a count of events, or a AVG(duration_ms). But if I have a custom tag I can't get accurate counts of that. And if I want to get distinct counts of values, I'm out of luck. Estimating that is an active machine learning research problem.