“Are the Shiites just incapable of performing as an Army?”
I think they’re mostly just unwilling to die defending Sunnis from other Sunnis. War will start in earnest when they reach Shia territory.
I see no certainty that a prolonged effort to reassert total Suni control will take place. There may be some effort but when they secure the natural Suni tribal areas, the effort will diminish
Their main task is going to be learning how to governwhat they have.