Critical Hits & Backstab

[Demivrgvs]

I'm sure I've read something about it but I can't find it anymore. Anyway, I hope there's someone that can easily dissolve my doubts.

 

1) A critical hit doubles all damage dealt including enchantment bonus, strengh modifier,...am I wrong?

 

2) Does the additional damage (like Impaler's +10) granted by a "damage type" effect (12) doubles too?

 

3) Strength bonus isn't multiplied by backstab, am I wrong? Is there some other bonus (like the one mentioned above) that isn't multiplied?

 

4) I've always assumed critical hits and backstab multiplier stack, is it correct?