Counseling is not an option while there is still an active affair going on. Most professionals, even on the "non-exposure" side of the aisle, would agree with that.
Counseling is not an option only because you say so. It can be an option if need be, regardless of what the “experts” say. Each situation is different and those “experts” screw up plenty, so don’t take that as some kind on guarantee.
I also have D18 overhearing her mother telling her brother on the phone that "I'm only going to go to counseling so people will think that I'm trying," and she's given me the same impression and I also confronted her with the "overhear" and she didn't deny the comment.
So let her go thinking that she is keeping up appearances for sake of the kids. What does it matter why she goes, so long as she goes? As the issues come out, it will become clear enough to her what the true problems are, but not going at all will allow the fog of denial to stay over her. Her statement about “trying” is just a deflection. If it were over, she would say it is over and she would leave. She doesn’t want to be the bad guy, to be blamed or have to deal with the guilt. This is one way she can justify her actions. But at the same time she is too scared to leave on her own, so she plays along with you by saying she will attend counseling.
Again, who cares what her reasons are. They are all deflections, delusions, justifications, etc. All those ideas will change over time with proper counseling, EVEN IF SHE IS STILL SEEING THE OM. Be sure your insistence that she stop seeing the OM first is not just another morality power play over her, or some way to feel like you are "getting even." That is only another form of escalation.