1) Joker is a cool, terrifying villain, but he's a wild dog. I don't think that anyone would trust him on the team. Nor do I think that he would want to join despite the fact that he has in the past? He hates schemers. Why would he want to join? He doesn't want to kill Bats--just torture him to the brink. He would probs betray the LOD.

2) Sinestro is committed to his own brand of JUSTICE in the universe. Why would he choose to work with villains. Would is make more sense for Atrocitus or Larfleeze to join?

Sinestro would and has worked with villains when it suited him. Atrocious is devoted to his own brand of Justice as well and Larfleeze is to single mindedly greedy. Joker wouldn't join willingly unless there was something in it for him, he was a veteran of the Injustice Gang after all,and I'm sure he wouldn't mind murdering some other heroes for the fun of it.

Dammit, I meant Atrocitus. Stupid iPad spellcheck