I talked with a person who watched a test screening and said that the Joker and Harley scenes distracted people from the main plot. He said that Joker's scenes were horribly edited and that Warner Bros fVcked up this film really really bad.

Besides the electric shock's scene, that relationship doesn't even have tension much less any kind of abuse. They could have at least introduced conflict between the couple. The abuse could have been showed indirectly. Anyhow , that was definitely not the reason that made the studio cut the film. The real reason was that the Joker and Harley scenes attracted too much attention and distracted the public from the main story.

Yep.
You can have a couple of 'flags' in there, obviously, but an entire storyline about a highly abusive relationship was probably pushing it a bit to far for a movie that was already pushing things.

Outstanding. Now all we need is a deck of cards.

But Slipknot punching an woman without a reason, is pg-13 material or what?

They took it out because Joker abusing Quinn/ a woman isn't too PG-13 these days.