score:4
According to a 2019 paper published in the Journal of Air Law and Commerce
The United States has regulations on tarmac delays and denied boarding compensation, but not on canceled flights. The EU has regulations on denied boarding, flight cancelation, and flight delay, but not on tarmac delay.
So there you have it, a sad negative to the first few of your really well formulated questions.
There was a 2013 proposal but it seems it didn't go anywhere. A 2015 briefing has this to say about it:
The proposal introduces some new rights. For example, the right for passengers to disembark after five hours of tarmac delay
This seems to corroborate the paper above -- current regulations do not cover tarmac delay because this would be a new right.