This is kinda like... fucked up?
OK, first of all, H is cheating on K. W also cheated on you with H. They both lied to you throughout the entire thing, and continued to lie to you right up until you confronted H about it.
You feel manipulated and played for a fool. These are reasonable feelings, considering you were manipulated and played for a fool.
But your biggest concern at the moment is "how can we all be together?"
Well, a good place to start would be W and H quitting their mind games of lies and deceit.
Have you ever actually met or spoken with K? How much of what you know about him comes through H? If her description is accurate, then I agree his behaviour is deplorable. However, bear in mind that she's been cheating on him for a year and a half that you know of, and who knows how long or how many times before that? I'm not trying to justify him being possessive and controlling, but clearly their relationship needs some work, and your involvement isn't helping with that. If she's not prepared to leave him, then she needs to get her shit together and deal with her marriage.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker