Ronstadt was originally supposed to do five performances a week, with Cohenour doing three. Before previews started, that changed to four and four. In the end (although this may not have started till after previews started), they added a third Mimi: Cohenour did four performances a week, Ronstadt did three, and Caroline Peyton did one. I think this should not have been a great surprise. I think probably no one should be singing Mimi even four times a week for an extended period, even in an intimate production, although Cohenour did it. If anything, the surprise is that there wasn't a third Rodolfo, and that they did not have alternates for Marcello and Musetta. I would think that if the production had moved on to an open-ended run, they would have needed a third Rodolfo and alternates for Marcello and Musetta.
You're right that both Ronstadt and Cohenour played opposite both Carroll and Morris. Peyton played opposite only Carroll, at least on the official schedule.