Or imagine this: every single purity route exists in some sorta parallel reality OR every single purity route is inside the bigger cycle, which is resetting once in a few years or something. Then the reset is happening and so Sensei relives his life with another person.
How do you imagine a purity route if it's going to end up being reset? that's sad and not the way it will happen.
Here's a question for you.
If you've lived a life and suddenly you were to forget everything, is the old you dead and you are now a different person, or is it still you?
You have already seen it before, different Senseis who are different from each other but originated from the same vantage point somewhere in the past. One likes poetry, another likes football, another is a remorseless groomer, all of them shaped by circumstance, and are so different that you can't call them the same person anymore.
I've already said it before, but our current Akira is the chosen one, I think he has inside of him the memories of previous Akiras that were reset but never deleted.
When you start a new game of LiL, everyone forgets everything, but your save file is still there and you can make them remember if you chose to by loading it.
Scientifically, if has been proven that a person can be romantically "in-love" with two people at the same time, but in our story, there have been so many endless resets that he has fallen in love with every single girl at some point in the past, it's all just locked inside until he remembers again.
So when Akira says "I love you" to a girl, he really feels it, because it has happened, he just doesn't remember, and the girls are the same too.
All of this amounts to Akira being one and everyone of himself at the same time. Now I don't know how Selebus will choose to do it (I have some ideas of my own), but you can easily get every purity route, harem and True endings to be cannon at the same time, if you can just SPLIT YOUR SOUL.