Well just to be clear in the associated story the ntr was really only semi-ntr and only happened temporarily before the FMC came to her senses and totally split from the college sleazeball and then dated and married our MC (and hooked up with our MC wayyy more overall)
The side story is actually about the FMC changing time so that that that semi-ntr speed bump never happens, even if it was only ever temporary; while making sure she still ends up with our MC.
So in chapter 16 that change via time travel was made canon and the bad shit has now never occured. Our MCs still have their happy ending together except now the route to it is event happier.
* Also given that time travel is now subtly canon, I feel confident in asserting that that dream our FMC (yankee girl) had of her future self being married to our MC and getting all lovey dovey with him and shit, was really an experience/vision from the actual future.
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@Darklight99 @agustD --- so the semi/temporary ntr has now been undone and canonically never occured, given that they changed time. So dont worry about that bad shit it should be mostly all good now >though Im sure there'll be some drama, I think any drama will be worked out, since our MCs (especially FMC) were able to work out and undo some of the most tedious drama that could've potentially occured.
>>And through the influence of our FMC our MC is becoming more ballsy all the time )
>> I think if anything that tie-in to the associated story only occurs because that associated story was the prototype for this main story, so our author wanted to make it all mesh <<
* It's also possible to view it as - both the bad shit, and most of the Time Travel, now never occured since the events leading to them have now never happened in the first place.