Oh, i dont think you have to necessarily start all over for the different playthroughs:
Also
So for instance - according to these posts - i should not have to many problems in my current Playthrough to get a Rosie Ending. But with my current 6 Rosie Affection and 12 Libido it will be likely the more depraved one.
It is my understanding, that i will probably only need to start completely anew (to tone down on her Libido and up her Affection), if i want to get the wholesome and gentle one.
TD1900s approach gives us - in my opinion - enough leeway in our decisions, to reach each desired outcome without the need to go back too far. Thumbs up for this philosophy.
Anyway, it seems to be a smart move to keep at least a couple of save slots at some crucial story forkings, to be able to shape your Playthrough to your liking.