How can H be with OW tomorrow if S is home from school - won't H need to stay with him?
As for all this other speculating about H and OW - it's really undermining your Act As If efforts. Think about what you would do if you had all the info you want? If you KNEW H was spending Saturday mornings boffing the oW - would that be the end of it for you, or would you just redouble your DBing efforts? If it's the latter - then just let it go out of your mind and redouble your DBing efforts.
If you would be totally done with the marriage if you found out he was still spending Saturday mornings with her - then hire a PI and find out, and be done with it. But all this ASSuming is getting in your way. You've listed a lot of positive baby steps. Maybe you should adopt Sage's old habit of listing daily positives?
Don't you think it's possible that this guy just is a really poor manager of his time at work and needs that extra work time?