I know they had a 'new game plus' system for a while, where once you got 16 resets you were given a free KT. I heard things got a little...interesting with KT's after that. A lot of unfavorable things were created so that system may have been abandoned. I created my own KT not to long ago, under the New Game Plus system but it's not nearly as crazy as some KG out there now. I feel like if you can come up with something interesting on your own without it being OP as all get out (And not a carbon copy of someone else's KT or Dust release) then you are fine to do whatever.
As for KG being trainable? On SL, no way. Though a few friends of mine came up with a theory that they can be trained with enough time and skill, but it was just us coming up with weird ideas and thoughts.
The way I see it, Mu was related to that ole fence sitter, because it makes no real sense why you need a blood relation to mix two elements, but not 3. Though that does not matter because it has never been shown one way or another. Unless Kishimoto is like "Oh yeah they are related." Then you don't need the same blood.