Breathe, in and out, in and out. Really deep breaths.
Don't worry about the mediation. Remember about thoughts and things. If you think it will be bad, it will. Let the mediators do their job. Besides, isn't this just the first step?
I'll repeat, breathe. In, Out, In, Out. Close your eyes and find your center.
The kids will be fine. IMO, it might be time to sit them down and tell them something. I don't really know what exactly because you don't really know what H is going to do, or is doing.
If OW, well, so be it. If not, well, so be it right now.
Get a hold of yourself, know that you are strong, and you will survive. It actually sounds like you've got a lot of other stuff to focus on right now besides H. I know it would be great to have him there for support and stuff but...
So H didn't come home and didn't respond to text. If something horrible happens to him, I'm sure someone will contact you. Until then, let it roll and be for you. Be for your kids. You can do this. Love you, you know.
"Acceptance doesn't mean resignation. It means understanding that something is what it is and there's got to be a way through it."--Michael J. Fox