rumBowl, there are too much discouragement to use it
1) No player must be adjacent to the Stabber. This will reduce number of situation when you can use skill
2) It's resolved as foul. So, it's mostly still the same Stab, but now with additional significant risk (33% of ejection)
3) ..and mostly the skill is still useless. -1 to roll from DP is not that much, comparing to the risk of ejection and taking into account there is no cheap players with Stab (in official rosters this will be only Assassin and Zara). The main feature of foul is the fact you can use assists, thus bumping your chances of successful removal of 1 opposing player to make up for the risks.
So, again, it's only relatively useful when you play against low AV teams. And you probably won't try to even use it unless you have bribes, you'll just be using fouls with enough assists instead, and that alone is more than enough of risk taken, you rarely will need even more. And usually few DE coaches decide to bloat their TV with assassins just in case they'll be playing some elves or flings. Assassins mostly a pure bloat against most of the teams, and very fragile (AV7). So it doesn't add much to the table, really..
May be just simply allowing to add DP effect to it will do, with all the rest left as it is atm? It will simply be a light buff to the skill, and Assassins. I would also consider giving them AV8 to the boot - while keeping their current value, or slightly bumping it. That would make them attractive enough option.
Another approach could be to give Stab some additional unique useful quality. Like, ability to override Block skill, for example? Probably because it's hard to fend off attacker if he tries to cut your hands with a hidden blade, or something. So, if Stabber blitzes, or blocks somebody with Block skill, on both down he negates it? Wrestlers still are skilled enough to evade knives, so they are unaffected
As slight debuff, you still could treat this "negate Block" action as Stab skill usage, thus no SPPs are earned from CAS this way. And, if it's still doesn't seem enough, Block negate action could be treated as foul (if doubles are rolled on either armor or injury it means ejection, but no assists are accounted for during armor roll in this case), thus you have a risk of ejection. So that would turn Assassins into sort of powerful ball sackers (and with their AG4 and A access they could get Leap easily and use it effctively as well, very interesting combo), but still with some risk included. If treating it like foul option is chosen, it could also be allowed for it to negate Wrestle as well, why not, it's already balanced enough by the high risk of ejection.