As for the second part, that isn't really relevant. As for the Gravemind, you do know what it does, right? The Gravemind assimilates all organic matter, knowledge, experiences, skills, and abilities. Even if the Gravemind can't use the Force just after infecting a Jedi, he could physically add the Jedi directly to his own organic mass. From that point I could get extremely hypothetical and just say he immediately destroys everything in his path, but he at least gains the ability to use the force, but with an infinite willpower and strength to back it up.