LESS RENT?!! My Third World mind can't process that! Here if someone said something like this I'd react like this: https://imgur.com/a/5wSMiIF

I have no idea, never been in the situation but you can bet I will be scouring the law books for my next move - America is very heavily biased toward property owners and not people who need a GD roof over their head, I hate it here

Depending on the temperature, probably at the week mark.

There is a legal regulation for this. But since I am a nice person in that regard I would discuss matters with my landlord and be friendly and try to come to a mutual agreement. Only in case the landlord is stubborn and stupid I would make full use of the legal regulation.

I would ask my landlord to fix it, not for less rent

one week in winter and that's generous, after that I'm cutting down to a sensible rate (300€)
perfectly legal too in my country.

After I flooded the apartment 🌚

I don't need/have heating, but I'd ask them about it first. If they have no intention of fixing it, then I would immediatly ask for a rent decrease