Thank you for taking your time to write this, I really appreciated it! I'll try and go by points.
-Choices and Consequences will be added in important parts of the game, like the Witch Quest in the upcoming build, where it has 2 paths and it will play out a role in the main story.
-These important choices will be carried out throughout the games until the end of the Era. Which means that you can "import" your save (I'll ask you questions in the beginning of the sequels) for you to have these consequences played out.
-Only these two topics above give me a lot of work, since they will accumulate, especially in the 4th and final game of an Era. Even AAA games struggle with it, imagine a single guy like me.
-I'm just one guy and I can't create hundreds of thousands of choices/consequences, even if small, throughout the game without hurting the development itself. Imagine creating small variations to every single battle, like you suggested, to all succubus we have planned, it will be a nightmare for me having to develop, play test, etc everything. For me this sounds wonderful but also unrealistic.
-The dialogue where you can treat Iole well and then in the next treat her bad, for example, is for the immersion of the player. The options to treat her well or bad will always be present, however is up to you to make your character consistent, it makes no sense to me treat her well and then treating her bad in the next interaction, even if the option is there. So yes, the dialogue will always be open for you to fit the character you want to build. But it's up to you to make him consistent.
-If you advance your relationship with these girls, by treating them well, in the case of Iole, you will unlock unique conversations and interactions, including hentai scenes. For normal interactions for the progression inside the plot, all options will be available, since I can't create a story based on how you treated every single person.
-Plot choices will be commented upon, however.
-The main focus of the game is trying to resist a fight with a succubus, their constant temptation, the risk of energy drain. This is the Age of Invasions, things will be completely different in each Era as we move forward. For example, in the present, succubus would have developed illusion magic, so they will become these true predators that only reveals when there is no chance for the victim to escape. Very close to the modern legend.
-Every single game follow a "box". A script if you may. To spice that I added the temptation events, that escpaes the formula of the battles, like the tribal succubus or the tied one.
-I don't know of a single game that doesn't have this "box". I prefer the formulaic approach, it's how I develop the game. From Monster Girl Quest to Lust Series, all of them have a pattern for the enemies you face, with some variations here and there. Then you must defeat them using acquired skills, items, equipment, etc. Not only it accelerates production, it keeps things consistent. For example, all girls can receive the same number of images, without one have more or less than other. Because the script is there, and therefor inclusion and development are objectively better and quicker, avoiding that I forget a image here or there for another girl.
-Believe it or not, I spend days to create these branches for the dialogue, the more options I give, the more complex and laborious it becomes. That's why very rare developers have them included in their games. The amount of work is huge to the point that if I add more of them I might burn out from the game and can't even look at it anymore.
-The way I chose to develop the game is the best compromise between including choices and burning out myself from the game.
-There is just so much in terms of including dialogue that I can add without becoming repetitive with a single set of images. That's why the generals have 2 set of images, so I can develop another scene.
-These new branches in option would result in the same thing, either the death of the protagonist or the succubus' defeat. Succubus in the world I built drains the man dry, they don't keep them in a eternal sex loop or carry them to hell to keep them as eternal slaves, etc. No the succubus here were build following my view on them, they are predators and they suck life, kill you and move on.
-More areas will be added to the next builds to expand the world and make it less linear.
I hope I addressed most of your points. Have a nice day!