I'm really into those types of scenarios. I read a lot of shoujo manga and that kind of stuff happens all the time too. Many times there's a childhood friend one-sided(ish) love guy thrown in there, with like about 10 years of relationship buildup, only to have Badboy McAlpha swoop the girl off...
Wish people translated Mashiraga Aki works. Some of the only good story-driven NTR (outside of just reading redikomi of course) with the added benefit of not having faceless fat bald guys.
Felt kinda all or nothing in the game. Either you netorare the girl, or she turns into some kind of dumpster slut (so it doesn't matter anyways). Blonde guy was the protagonist for like 1/3rd of the game and the spin-off was from his POV too.