My interpretation of what the 3rd ending was meant to be was Ken deciding to make the girls leave the mansion by themselves as he stays behind with his grandpa to solve the problem, but the problem persists creating a paradox where the girls go back, a space-time device ona-hole is given to the old man/ or a dimension portal pops up in grandpa's closet where he can leave the mansion connected to various houses in the village, and still NTR's the girls away from Ken without him finding out, only learning about it at the end.
Meaning typically, NTR cannot be avoided.
edit: Upon re-reading what I said I felt I should explain the ona-hole reference thingy. It works like a fleshlight and the girls are magically connected to it, allowing grandpa to do various lewd things to the girls. Except it's not an ona-hole/ fleshlight in so much as it's a device that would've allowed him to still mess with the girls in a way that make them get corrupted into grandpas harem.