It is straightforward though: NATO is an organization for collective defense. The set of territories covered is well-defined, but does not include all territories claimed by each individual member states. For example Hawaii is not included, nor is the Falkland Islands, which is why NATO was not invoked in the Falklands War. But Greenland is included.
A response to an attack is decided on by the NATO council, not by the individual member states. So unless NATO is dissolved (which could of course happen one day but which is certainly not in the cards currently), Greenland is defended by the NATO alliance.
Taiwan is of course not a member of NATO so that example is irrelevant.
No, because Greenland is part of NATO and none of the member states disputes that.