Well I think Simon will tell Carly but he'll say that was the closure and that he's ready to move on. That is what I think. (Even though I don't think he is) Katie will always be #1 but since she wants nothing to do with him he is trying to move on. But he won't be able to.
I don't know if Katie will tell Mike. I wouldn't if I was her. It's over and if she has any intention of trying to make her marriage work then she won't spill the beans. Of course I think Mike will find out at the worst possible time and depart but that's just my thinking.
I think Carly will be wise to Katie now and how jealous Katie has been. Yes it's wrong for Katie to be so jealous she's married. But then Simon is just as selfish as Katie so IDK. Plus Simon has done a lot to Katie in the past. Simon has never truly left Katie alone. Now the moment he will and try to move on Katie will either end her relationship with Mike and try to pursue Simon or she'll get pregnant by Simon. Whichever. IMO