Wow! I can't thank you enough for this post! This is the level of brainstorm I could only dream of.
On Corruption: The point is that corruption was, is and probably will be my most favorite genre in eroges.
Making NTR unavoidable will basically make gameplay mechanics not worth learning - what's the point of trying to win in the game if ultimately it all ends the same, right? On the other hand, making NTR avoidable leaves you one of these two general choices:
1. Think, explore mechanics, get good and save Aster for yourself
2. Play the game of giveaway or lose naturally. Consequences of the latter option won't even be considered unexpected or frustrating - you know what genre of game you're playing.
I believe that the middle ground is not so unattainable - a game can respect players' freedom, give them choices, reward for their effort and "punish" for not playing it the way it wants them to play. If you put in your effort and succeed, you're satisfied. If you come just to get punished, you don't even have to put in that much of an effort and even then succeed - you got what you wanted, you're satisfied. This is how I look at it. In the case of unavoidable NTR - doesn't matter if you try or not - you always get generally the same result which wouldn't be defined as "success" by every player.
Also, I don't insist, but suggest you to read through my posts on F95. I hope you'll like my concept more.
Suggestions:
On Mentor's Identity: I thought of that too. He's already casting Dark Orbs. Maybe something like making him sort of an universal mage who also have mastered some sort of fantasy pagan/voodoo/forbidden magic which would be necessary for Aster's ascension. And, of course, he would be the last man alive able to share this knowledge.
On shared Mentor: This is a great idea. Honestly, I like my image of Mentor which doesn't really care if the Hero finds everything out. But I will think about making him just sadistic towards Hero. This can create some interesting situations for characters, that's for sure.
On side girls: I want Hero to be entirely loyal to Aster.
On level system: That's interesting, although regarding levels to be considered "final" or "high" - I think that the concept of levels is too abstract for a world which is not an in-game made-up VR MMORPG or isekai. If I make an implementation of character levels, it will be available just as an abstract metric for players to measure their progress and unlock new skills, not that the term "level" will ever gets voiced by characters.
On equipment servicing: Another great idea! I really like the idea of forced return to cheaper options. I think it fits well with the hostile nature of the world I plan.
On alternative use of torches: Sounds interesing too. I will think about it.
On being always too close: I actually want to give player an option to split up with party on forking paths. Ever since I started to work on this game. Of course player, as a Hero, must have reasons for that - which will be addressed later.