Is it common for apartments to have shared extractor ducts?
No, but happens in crap countries like Croatia. Its sad, but I have to deal with it.
Turning on hood with relay like that would make it difficult to use when we are cooking (cant set the speed and need some kind of switch available). I was thinking I should solder relay on lowest speed, but that might cause additional issues.
Temp and humidity might work, but im not completely sure since its quite small amount of air coming trought. I should probably get some sensor and see output