Darcy, to me, alcohol seems quite straightforward, here, i.e. it gets priority processing, so the liver/body indeed stops or slows the ketone production for a while.
In reading the first few posts at that webpage you gave the link to, it’s mentioned that it may take time for Metformin to build up in the system, and/or that people had to increase the dosage in order to get into ketosis. That seems strange to me - if the concern is Metformin preventing or reducing ketosis, i.e. lesser amounts of Metformin kick people out of ketosis but larger amounts let them back in? Seems complicated and counter-intuitive to me. Yet I guess stranger things have happened…?