Does anyolne here feel like I'm over reacting to her having OM stay the night after a great Marriage session?

Well, maybe you needn't over react, maybe it's more a matter of accepting that something's rotten in the state of Denmark, you know?

Doesn't this tell me something that she would do this on the same night of our MC?

Yep.

she must have me and MC fooled huh?

I dunno. She might be hiding stuff from her counselor. You know, just cause she's seeing a counselor doesn't automatically stop the A. But continuing the A while going to MC sure indicates that the counseling may prove ineffective.

I get the sense though that your C does see it, and is trying to get you to move push to shove, recognizing that this is not going to go anywhere you'd like it to right now.

while admittedly there is nothing more I would like than to find the one man that I will spend the rest of my days with... but I've been fortunate to have experienced it a few times in my life

Geez, how many men do you have that you're spending the rest of your life with, anyway?