Confrontations are always nerve wracking. No way around it. You seem convinced about this course of action. You are correct, this doesn't really fall under DBing. DBing would be to detach, GAL, 180, and be the best you that you can be.

Just try to stay calm and emotionally detached during the discussion tonight. Do not beg, plead, reason or try to guilt him. Be matter of fact. Lay out what you know, let him respond. Make sure he is aware that you have boundaries and lines in the sand. If this move in with the OW is a bridge too far (in other words, there is no coming back from it) make sure he is aware of that. Likely he is keeping you as plan B. 30 23 is a big deal. He is established and she is just getting established. Her life is likely in a very different place from his. There will be frays in that arrangement fairly quickly, he probably realizes there is a high likelihood of that all falling apart and want you waiting in the wings.

M(53), W(54),D(19)
M-23, T-25 Bomb Drop - Dec.23, 2017
Ring and Piecing since March 2018