WE GOT A BACK TO THE FUTURE SCENARIO! Just don't make em fall for her son, would be nice if they are just friends and then he helps em get together. But this is a manga and those scenarios are a dime a dozen
Gaku has been changing shit nonstop, i doubt it even matters if his parents hook up. I think he must be on a different timeline not so much in "the past" of his timeline, but idk time travel stories are usually full of weird logical inconsistencies and holes even when they are good.