I don't understand the point....
Zoey arrives in San Diego in the summer, in early fall at the latest, if her grandmother died at the end of the previous school year it doesn't create any problems (we actually don't know how long the grieving lasts and how much time Zoey takes to decide).
Mc has an entire (or at least almost entire) school year ahead of him, which of course lasts less than 12 months.
That timing still seems off. The MC says he'll know trigonometry "in spring." If this takes place at the end of the MC's junior year (i.e. the spring semester), that's an odd way to phrase it. Also, it would mean Zoey must have flunked trig in the MC's
sophomore year (or at least the fall of his junior year, if we assume trig is taught separately in both semesters), since otherwise she wouldn't have known for certain she'd failed yet. That's a long time for her to be waiting around to make it up.
The scene makes the most sense in the fall of the MC's senior year.
the question of their first time is binding for Patreon, it can't become so no matter what we are told, we can also think that MC turns 18, they have sex, Nana died, and Zoey shortly after leaves for San Diego. even in this hypothesis from when she leaves to when she comes back a year passes, she leaves with MC who has celebrated 18 and comes back who has already turned 19 a while ago
We know the MC's birthday is in roughly the third week of the B&R fall semester because he turned 19 in Episode 6. We can say with certainty that any sex taking place before his senior year in high school would be before he turned 18.
Thus if we want to stay strictly above board, that likewise forces the Interlude into the mid-fall in the MC's senior year. The timeline fits all the known (retconned) data points. The only issue is that it means the MC's "FWB" relationship with Zoey lasted at most 2 months. But that's a side effect of giving the MC a sexual history prior to his 19th birthday. There's not much room to work with, and Zoey needed to be long enough ago that the MC had started to move on.
DPC should have had the MC take a year off to save up for college (so he'd have turned 20 in Episode 6), but he probably wasn't thinking through all the implications of his backstories when the game got started.