Intelligence agencies were quite clear early on that President Putin was intent on launching an invasion. The risks with ambiguity signalling...if you're confident that the invasion will be launched, and you're ambiguous in your signalling, there is a real risk that failure to take the action you haven't announced will be seen as a sign of weakness.
There has been a determined effort by NATO to avoid putting NATO and Russian troops in contact with each other. The concern with the no-fly zone was, first of all, that we couldn't expect the UN Security Council to authorize one. Russia has a veto. Absent UN authorization, it becomes more difficult. A no-fly zone, of course, has to be enforced. We would almost certainly have seen a situation in which NATO troops and Russian troops were interacting.