The issue is: if Nato do nothing and sweep the 'accident' under the rug - Russia reads thats as Nato unable/reluctant to get involved. Hence the probable use of chemical or targeted nuclear weapons against Ukraine which would be much more 'doable' with confidence if they feel Nato are squeamish about involvement.
Nato moves in and stands by the article of declaration. WW3 is on the cards, and Russia can signal it was just a stray but by then it might be too late. Russia are completely ruined economically, militarily, they have run out of road and roubles. Putin can not bear to lose. Remember the caged bear analogy.
Nato fudges with Article 4 and hopes to god Russia stops there (It won't, surely we know that now)
At this moment the back channels of communication could not be more important, but in my view it is almost too late. This is more serious than anything we have seen in the world in several decades, and I am pretty sure most world leaders are not buying that it was just two stray missiles.....