Sorry if I wasn't clear, the successes would have added dice to your dice pool, but still 1 success.
I break it down like this: Dex 2 + Larcany 1 + WP 3 + 2 dice for successful stealth roll -5 modifier= 3d10. Pulling from the 10 again pool to get the rest of the dice.
EDIT: Karl is now down to 0 WP.
EDIT 2: So here's what we are going to do with the grenade. We are going to use the Chronicles of Darkness book "Armory" which has grenade rules. Karl, who has to have moved from outside the room, where he ejected the Dominated vampire's clip, all the way back to Prince Gorman is going to have to have used his entire movement for the turn to get to the Prince where he successfully plants the grenade. I am going to use the optional rule for grenade timers, where I am secretly rolling a D10 and subtracting it from Karl's initiative to give the grenade an initiative. It will go off on its initiative. This will HOPEFULLY, give The Prince time to run away from Karl before it goes off.