As you say, it has been the premise that every single girl in the game would be fuckable (except Sara). Not that I'd necessarily agree with it, but I don't really mind to be honest. Maybe that's because I don't really care that much for specific scenes in particular and I like the variety.
Though I'd agree that they probably should be a little more carefully with the number of new characters they add. DC mentioned some time ago that they added the mafia girls because they thought the final warehouse arc would be too much of a sausage fest without them, but if you asked me that wouldn't have been an issue.
Yes, that's more or less what the game is indeed about. MC will impregnate every single girl in town*, including Roz
*(excluding Sara, as said)