Regarding Mark, you’re right—that was a mistake.
As for Ronny, FMC simply tells her fiancé that she met "a guy" (Ronny), and it's up to you to decide whether to push for more details or let it go. Her mentioning Ronny is a slight stretch, but it's necessary because I need MC and Ronny to become friends in some way. Ronny needs to be present even when MC is around so that different story paths can unfold. MC will have the option to see him just as a friend, share his fiancée with him, or let her spend time with Ronny on her own.