The main quest is complete, so if you want to, you can jump in whenever you like. Also, saves are retro compatible 99.9% of the time, so you generally won't miss anything by starting to play now (the only exception being if I add new content to old paths, which seems unlikely at this point).
On the plus side, the farm and the vampire don't instantly lock you in their respective paths, you need to go there for a while before the effects on Erica materialize (that's my way of giving a buffer to the players, as you noticed). Yeah, there is a limit to what makes sense and constantly asking players their input is also bad for the flow of the scenes, so it's a balancing act. You can however play the entire game without seeing a single sex scene, if I remember correctly, so I hope that's good enough in the end.
Thanks for the bug report! This actually never came up before, believe it or not. Since the schedule was introduced late into the game, it's likely that there are still issues out there. If you know of more scenes that do this, tell me about each one individually. Sadly, because of how the game's logic works, there isn't an easy way to find and replace all the bugged ones, so I'll have to do so manually whenever I hear about one.