Regarding the whole Maul/Greivous thing: What if Sidious viewed Maul as a failure after his supposed destruction on Naboo and dropped him as a Sith apprentice? Maul would undergo years of salvage work, and after a bit of bond-o, we are left with Greivous, a shell of what he used to be. Maybe the overall lack of a body would also account for him not being able to tap into the Force. That alone would result in being re-assigned by Sidious. This would be just a step beyond what Vandal just suggested.