what will happen?
"Endless infighting like with the Sedevacantist groups, splitting and splitting until each group is utterly insignificant in the big picture."
Why? Because the anti-Accordistas (or Pro-Lefebvre faction) are united only in their lack of obedience, their foulness of disposition, and their lack of virtue?
On the contrary!
I would expect them to hang together rather nicely -- after all, they've done so for years, if not decades now. All they want to do is carry on the mission of the Archbishop and carry the torch of the Catholic Faith, uncompromised.
The fact that Bishop Fellay and his clique have decided to betray has nothing to do with the dozens (or hundreds) of faithful SSPX priests, as well as the faithful Bishops.
Now if Bishop Fellay had done NOTHING to change, and a bunch of priests left, you would be right about splitting ad infinitum. But the fact is that Bishop Fellay HAS made a fundamental change to his position vis-a-vis Vatican II and the Crisis in general.