This sentence made my night: I’m more than thrilled that once, in 10 million tries, a 5th level assassin met the one 14th level target that rolled badly enough on HP to be assassinated. That guy is a legend somewhere.
I like it. One question though…in your damage table, it looked like you were multiplying the damage modifier by the backstab. I’m pretty sure that all modifiers are added after the dice. So a 14th level backstab is only 10-35 damage (barring magic and strength mods (1d6x5)+5).
For what it's worth, the Assassination hijink difficulties are meant to include the difficulties of e.g. sneaking up on the victim, finding him when he's alone, and so on, so they ought to be lower than the chances of scoring a kill in and of themselves.