I think we have a misunderstanding here. Accomplice's procs increase your Stealth level, not generate Upper Hand charges. The idea is that your accomplice is keeping the target busy, making it easier for the target to lose track of you. Stealth level isn't about being invisible so much as the enemy not being able to track you in the chaos of battle.
Upper Hand charges are only generated from your attacks (at lv80 its 10% chance for all weaponskills, 15% chance if executed from the rear of the target), Runaround, Underhanded Tactics, and Sneak Attack (provided you use it while under the effect of Minor, Major or Complete Obfuscation). You do bring up some good points on the frequency of Upper Hand proccing at a reasonable rate, so I'll look into raising the proc rate or doing something like allowing auto-attacks to also proc charges of Upper Hand.
True. I guess I could just allow suspension of disbelief to come into play. I was concerned that if I did something like "Steal Attack Power", someone would be bound to ask why the mob doesn't have a penalty to damage dealt for the duration to show the THF stole that target's attack power to boost themselves.As for the DET increase, why not use a more generalized damage increase ? The issue is that every stat curve gets changed when you level up and when there's an expac released. So that would be a nightmare to balance, as the spell would need a change at every expansion at least in order to stay somewhat relevant at every level which is something really important. I would suggest going for the more straightforward route of "increase your damage dealt by 2%" maybe more, maybe less, depending on what you want. Most damage class do not stack DET that much too, so depending on the BiS this buff can be both amazing or completely useless.
Once again, thank you for the feedback.