I dont think any one type of presentation is better than another as long as the game is developed consistently with the POV in mind. In most games, adult or otherwise, you are generally playing the role of a preset character with some roleplaying element thrown in. Its probably much easier to write a compelling plot with this type of approach since it opens up any type of POV, narrator, and scenes where the MC isnt present. I think a lot of people would still consider this as "you are the MC," but maybe you are thinking of a more strict definition.
A big problem with the MC being the avatar of the player is that there is no character creator for these rendered CG games. This plays kind of a big role in the immersion i think and limits the POV to 1st person/3rd person backside view.
So if you want to frame the story as if the player was really the MC
1) 1st person predominantly with some 3rd person view form the back during sex scenes
2) Whether internal thoughts are acceptable depends entirely on the writer. "Oh my god, my sister's ass is so fine." "Im so horny right now." This kind of stuff is crap really. If the writing is good, a mix of actual thoughts and narration of MC's feelings do not take away too much from the immersion and adds to the story if used sparingly and appropriately.
3) You shouldnt be able to read other characters thoughts in this type of game, but you could read their reactions. "You see her smiling." Or "She seemed frightened by that exchange."
4) This one can be ok if it adds to the story. You can still have an omniscient narrator but frame the game in strictly 1st person POV i think. I guess this depends more on how you want the plot to play out and what kind of story you want to tell. Tbh its probably hard to tell a good story when all you have is the POV of the MC. cRPGs like baldurs gate or pillars use omniscient narrator and that doesnt make the game any less immersive. Which brings us to the biggest hurdle for MC being the player's avatar....
You need ALOT of text variations. Notice how in pretty much all cRPGs, the MC doesnt say anything unless you choose what to say. In these games, you also have companions most of the time who can do alot of the talking for you. Its probably nearly impossible to do something like that for a VN, but there should be at least one dialogue choice prompt for every major conversation which drives the rest of the dialogue for that encounter. A very robust morality/karma system is probably needed along with relationship scores.
I dont know why I wrote so much since I dont know anything about game development though lol.
