Thank you for taking your time to write all of this out. I have to disagree with your 9.5 rating of this game, though https://www.dlsite.com/maniax/work/=/product_id/RJ269335.html.
I gave it a 9- not 9.5. It was lacking in some aspects but for sheer ambition and unique way to acquire scenes from a male perspective- the phonecalls/SNS system/ voicemails were all done well. But in the end it misses out on some of the most important NTR aspects- like actual development between the heroine and protag-which I guess the cave flashback systems are there to build but the fact is that present day Ryoka never decides to show that at all. It's the all too common trope of the protag just waiting too long to show his feelings while heroine can only wonder if he even cares. It would have been nice if there was 1 or 2 of the NTR routes that happened after confession- or if the heroine was just closer to the MC. Originally her sister was going to be the protagonists sister- but that was changed. I noticed there is a lot of people asking for closer relationships so maybe developer will make a true NTR game where you start as a couple or bf/gf and go from there next time. I really like the art though and it just needs something more for the MC to do- maybe a skillbased minigame to work up stats that matter more for scenes than just the true end and to add to the effect that you're fighting against something with your own power. Rather than just ignoring heroine as she messes around for a month. But it was the devs first attempt at a game like this in that engine and I think they did quite well all things considered. They listen to feedback so I think they'll improve.