I say [name_f]Marin[/name_f] = [name_f]Mar[/name_f]-INN and [name_f]Maren[/name_f] = [name_f]MARE[/name_f]-in. [name_m]Don[/name_m]'t know if that’s “correct”, but that’s what I’ve heard.
However, I think this is just one of those names half the people you meet will pronounce just a little bit “wrong” (differently than you do) no matter how you spell it. We encounter that with our daughter’s name, but we expected that when we named her. It’s not a big deal to me. I usually don’t even correct people unless they ask and just chalk it up to different accents.