1: no more threat rolls, if you roll threat range, and your current roll also hits their AC, it's a critical hit.
2: Any effect that reduces threat range stacks, but it only reduces it by 1. 5% greater chance of critting, multiple effects of the same type won't stack however, like multiple keen effects.
3: On a critical hit base weapon damage is maximized,
a x3 weapon will deal max x2
and a x4 will instead deal Max x3
I have used these rules in my games and they have worked out wonderfully
speeds up gameplay and it makes critical hits more meaningful.