Yeah, the two paths are mutually exclusive: Sunny catches you in touch with Mel, and then it's over. So you're gonna have to choose one or the other. About the solo path with Mel, it's not my favorite path...Sure, it's pretty depraved, but there's no romance there.
But in general, yes, the solo path with Mel (without Des) is quite possible, and Riley can ask her not to invite anyone else (no men).In general, there are several options of events, so if you don't want to miss anything, I recommend you to install
URM and go through the whole game with it.