It's more I just don't know when I've experienced them all and it's safe to move on.
When the quests and side and/or optional quests are all marked as completed, you've experienced them all.
Are there relationship scenes that only happen in specific quarters?
Some of them and that would be your room. I literally pasted that from the code of the game.
I end up not going on dates ever or asking them to sleep over because it seems like most of those scenes happen in the morning when you wake up in your room alone. Worrying about scene triggers is actively making me not want to pursue the girls in the manner I would prefer to.
The relationship/main events have priority over side events. You don't need to worry about the side event triggering or blocking a main event. The requirements don't require you to be alone. If they need to talk to you alone, they will ask for the room. The only presence requirements for these quests are that the person in question is not in your room with you. The main events are also not randomized. They are guaranteed to be evaluated if the conditions are right. Basically, if you're living day-to-day like an actual person would in that story (i.e. attending classes, doing training, etc.) you won't miss any events. If you're just trying to solve an equations so the girls can jump on your dick, you're probably playing the wrong game.
I honestly don't know how you're having so much problems. I'm on my 5th playthrough and I've been able to achieve poly for all of the playthroughs. It feels like to me like you're taking the approach of "If I do X, have Y and grind Z, they will like me. They have to!" Maybe I'm misreading it. But that's not how this game works nor is it how real people work. Like take it from a programmer, you can't guarantee someone to like you and be with you just by checking check boxes or following an equation. People need more than that (just in case I'm being too subtle, I'm trying to drop game hints here). Sometimes, a girl who's detached from themselves emotionally needs another woman to talk to who IS attached to their emotions *hint* *hint*. Sometimes, it takes a friend's help for someone to reconnect with their human feelings and carnal desires. *HINT* And, maybe, by trying to grind events and stats, you're missing out on the life that's happening around you. Like, how well do you Laura? Do you even know what her quirks are? Does she feel comfortable enough with you to talk to you about them?*HINT!!!*