but I cannot give up
And no -one is asking you to do just that. This is not a "kick them to the kerb" forum.
However as Acorn states - staying attached to an idea creates anxiety and it is that anxiety that keeps people stuck.
You say your D is pushing your wife to therapy. It's natural for them to want that.
Everything I have read and heard indicates that therapy can shorten MLC. It makes sense. If someone gets help to sort thorough their issues they should come out of it faste
The thing is - it makes sense to logical people.
MLCers are illogical. If you have PTSD or have had some kind of trauma and it all comes to a head but your behaviour around the family remains consistent and true then therapy is often a good thing.
However, as you say your MLCer is deluded, therefore the outcome of any therapy might make it worse for the loved ones. The therapist can only work with what the client says about themselves. And so there is the high risk that,even if the MLCer goes to therapy, (and very often they refuse because " there's nothing wrong with them!") the outcome will have the opposite desired effect.