I struggle with the concept of cake-eating myself. It feels very transactional to me (I'll only be nice to you if you do what I want you to do!) and totally opposite to the way I think love and care work.
For me, I guess, a helpful definition of cake-eating is where you give someone something that you don't want to give them - when it's part of a secret manipulation tactic and isn't given freely.
If I do some wifely service for my husband (I don't know - like cooking and looking after him when he was sick) knowing full well he's an abusive so and so who doesn't respect me, I am feeding him cake. I am giving him the benefit of my care and attention when he offers little in return. But it isn't about him - it's about me. Because when I did that, it was clearly a pretty manipulative move on my part. I'm angry with him. I don't like him much at the moment, and instead of withdrawing and acting with honesty - making my actions congruent with my feelings and the facts of our relationship - I pretended to be a nice wife. I stuffed all that down and played nice in the hope that it would make him be nice to me. It's grim and unhealthy.
If she was determined to leave she would. She's legally, probably, entitled to be in the house. So perhaps you just work on making sure that your actions are congruent with your feelings, and anything kind you offer to her is offered freely, whether or not she is cheating, whether or not she wants to divorce you.
I no longer share any finances with my H and I don't financially enable or support him in any way. We did that while we were together because we were working towards a shared future. Now we aren't working towards a shared future, so my responsibility is to support my children and myself. That feels honest and realistic to me, though obviously to him it feels like punishment (he still wants me to pay some of his bills) his feelings about what I'm doing or not doing with my money aren't my concern.
