There were actually two reasons why this story was titled "Dirty Pool", and you touch on one of them with your question about Carl. Kelly played an innocent, helpful, well-intentioned dirty pool, but Carl played a dirty game period, with no concern for what Duncan wanted. He's lacking any kind of moral character, and it's clear he's used to lying. He kept Duncan off balance by using their friendship, but that betrayal was beyond the pale. Duncan never loved him in a romantic way and never would. Some just can't accept reality.
Kelly and Martin did take a risk, after staying out of it until it became unbearable. It was affecting their son, Noah, and that made Kelly feel he had the right to do something.
I've seen first hand how dedication to work can cause a relationship to implode. If you don't have balance in a relationship, it's damn hard to avoid resentment form happening. Yeah, I think they've learned all the lessons in that regard they need to, and Danny will keep them honest. I agree... I'm all about it being better to give than receive, and nowhere is that more rewarding than in a relationship.
Thanks for the support and the kind words, my friend. I must admit this was probably the easiest story for me to write... it practically wrote itself. Cheers!