You have to finish it entirely -- you'll know when it happens (a dramatic UI change happens, after the green door event). Then go through a day and start New Game Plus (the option to clean up in Fiona's room in her endgame). You can then start it in the afternoon in the greenhouse in your new game+.
And yes you have to be the victim throughout the entire arc that's required. No revenge without a thing to get revenge over!
.....or just download this "dummy save" that lets you access the new content with basically no friction -- you have a lot of money and your skills needed for the new content are already maxed out: https://mega.nz/file/vc8QyADb#s-zlbYJjjB7gu9K2vfLxfhe6Pkt1r1PZjCyXY10QLos
(Click Load, not Import, for this)