You've commented on the question of when to do Retrouvaille. Having been there myself, I would say the sooner the better. It cannot be too soon to go to Retrouvaille. On the other hand it can be too late.
They do not accept couples with an active affair. That's why Lester should do it before she really strays. While she's on the fence. Once she decides she's in love with OM and would rather lose Lester than OM it is too late. At least until that phase passes. When OM is gone again, then it would be time for Retrouvaille also.
I understand your desire to strengthen Lester's power within the marriage. I agree he needs to give her more direction, not vice versa. But that has no bearing at Retrouvaille. They focus totally on communication and correct problems there. Everything is 50/50. Same amount of time to think, write and discuss issues. Equal weight to both points of view. That by itself would be strengthening Lester's side since right now we only hear her.