So...basically, high damage means more hitstun. And the strength of a move also affects hitstun.
Completely right.
Every move has "hidden" values such as Base Knockback (BKB) and Knockback Growth (KBG) and among other properties, such as angle and more. As far as I've investigated (and understood): BKB is like raw launching power, and KBG is launching power scaling (the more damaged your foe, the bigger the scaling).
Even similar moves with the exact same amount of BKB and KBG,
an attacks which deals more damage will deal more launching distance and stun frames.
Hmm.....at what points would an u-tilt upair be a guaranteed combo? Because upair can be a fairly reliable kill option.
Mmm... tall order... It's heavily dependent of your foe's weight, damage and DI. You're asking for the complete event of a hit (Distance and angle traveled, Frames stunned (true combo or string), fastest input/frame chains, etc). I can only say experience will only tell you.
I mean, this whole event can be calculated, but it will take me a lot of time. Sorry, I can't help you for now.