In the final version of the game there will be 2 unique Grab Kills per character ;)
Maybe more depending on the success of the Kickstarter campaign !
Currently they are not random : a different Grab Kill is triggered whether you are on the floor or in the air. But if we have the budget to do extra sequences they will be chosen randomly.
We also plan on making different moves / idle animations for each character. You're right about the fact that this kind of details can make a big difference on the overall quality of the final product ;)