She is going slow, testing the waters and you just need to go slow.
I think there needs to be a clear "Boundary" here. My DB Coach and Sandi2(both women)basically stated she has to feel a sense of loss. She can't have BOTH.
This doesn't mean you be an A-hole, but SHE will not respect you if you "date" her knowing that she has someone else. Period.
This also means you can't go "rage" against her, you have to show yourself as the better option, but dating her while she might be actively dating OM is just not going to work.(In other words, keep working on you and doing DB)
Sinclair, YOU ARE THE BETTER MAN. You show her this by going out and and making her WANT to be with you by NOT accepting an open marriage and working on YOUR faults in the M. Your work here is being the BETTER YOU. You want to leave her with a sense of "Why am I with the OM, when I have this guy?"
If she is actively seeing this other guy and TELLING you she isn't, you have the right to verify this. YOU cannot compete with this guy AND respect yourself the next day.
You need to keep DB'ing, but not as a "second" option. Even MWD tells you how to obtain this respect and if you don't achieve it you do have the option to take ultimate action.
^^^^^ABSOLUTELY^^^^^^
In Divorce Remedy, in the chapter on Infidelity, MWD makes the need for this boundary very clear.
It has the potential to be "wonderful news" AFTER this boundary is in place and being respected by your W Sinclair. I hope for you that this happens. But, until then, you would be wise to follow the approach that Faith outlines above.