OC, Would you still describe his contact with the OW as an EA? It sounds like emotional blackmail to me--like a fatal attraction.
I think the approach you described of building connection in the M and not getting in a power struggle over his contact with her is a good one. Your H may well have gotten himself tangled-up with someone he cant't easily dispose of. Hopefully this will teach him a lesson about the mental health of those who choose to have an A.
If she's that desperate, she has no power over you and your M. Keep building on the positives. His R with her is going to self-destruct. He will need to learn how to detach from her psychiatric drama.
CL
CL 53 W 54 M 20 yrs. 03'-09' Separation + Old Patterns + GAL 10-14' Piecing
"The Master allows things to happen. She shapes events as they come."