None of that explains why he is tolerating her to this extend and going along with her 20 iq "schemes" like he is getting blackmailed.
Because he doesn't want to get too caught up in her pace. Remember she already knew about his relationship with his sister and by her own omission she's been in "love" with him for a long time. She was also able to successfully move in next door, introduced herself to his stepsister, set up a near unavoidable situation, successfully gaslight the sister into thinking she's a burden/she wants to repay them back for being a burden (albeit in a more twisted way) etc. I would tread very lightly around someone who can manipulate situations that well.
Short answer-
To paraphrase Jesse Pinkman "Look the MC is just a guy....Mochizuki, she's the devil. She's smarter than him, she's luckier than him, whatever you think is going to happen the reverse opposite will happen!"