Apprentice Training in Backstabbing does nothing other than adding a flavour tooltip to the skill. You never bypass Damage Resistance when backstabbing. This can be easily verified by attacking someone from behind before and after casting Weaken on it. Investing in the skill doesn't add 1 damage per point invested. Instead, when backstabbing an enemy you deal your Attack Damage + (3/4 * Backstab Points) each time you succesfully hit your target. Equip a Railroad Spike or a Rusty Dagger for diminished variance and you will always deal 16-17 damage with the skill maxed assuming there're no strength modifiers. In addition, when attacking an unaware target from behind you do indeed gain a 5x multiplier to your Backstab Bonus so an attack on the same 0 Damage Resistance target as before deals Attack Damage + (3/4 * Backstab Points) * 5. Calculation damage post mitigation is easier, since all that needs to be done is substracting the Damage Resistance from the values obtained above. One thing that I would like to know, on the other hand, is when and how do you instantly kill foes. Sometimes, a lot of times to be honest, when you backstab an unaware target you land CRITICAL HITS that maim/instantly kill it or at worst give you yet another damage bonus (+50%/+100%) I'm completely guessing here but perhaps it has something do with the To Hit calculation: if you score a high roll then you land a Critical blow on the victim, who then has to pass another check (Constitution?) to see if it's killed/disarmed/crippled/etc and which damage multiplier is added to the backstab bonus in case it didn't die. Could this be why we keep seeing CRITICAL HIT floats with no damage displayed and the enemy dropping to the ground all chopped up?
I'm not really sure how critical hits are calculated to be honest, possibly a high roll as you say. I'm also not sure on what the effects of a critical hit are - flat damage or something else (insta-kills as you mentioned). With all the enemies you critical hitted were they on full health initially or lower health? It could be that if you do enough damage to an enemy with full health to kill them in one it doesn't show the damage. I never use damage floaters anyway so I wouldn't be the best person to ask.