Um, most of what you listed are problems of the individual and not problems of media but I will only address your statement about it being able to be made into a Teen game.

I am of the mind that art should be at whatever "rating" that is intended by the artist for their art. That is to say, I think the game should have whatever rating it is going for. If the developers want to add sex and violence, all the power to them because that is in the vision of the game they intend to make and lies in the vision of many (but not all) of those supporting the game. Watering something down to seek a larger audience is actually a little bit of an issue in media in my eyes, cause it is typically not from the perspective of the artist(s) but businessmen. Often times the lines between a PG-13 movie and an R rated film is just how many times a character cursed. That line can similarly be blurred with video games. Often times Teen games have violence and sexual content that is more than their M rated counterparts but the only difference is that there isn't blood with that violence, or characters are more clothed while being sexual. The lines can often be arbitrary. Also often times watering something down just to reach a Teen rating can be detrimental to the experience. An example that comes to mind is Mortal Kombat vs. DC Universe, whereas the origin series of Mortal Kombat is known for its violence and gore, it is expected with the series, yet they made sure to push for a Teen rating so they could get more Teen comic and superhero fans. However this actually made some fans of MK unhappy as violence was an expected part of MK, it was a major part of its aesthetic. This can go in the other direct, sometimes adding things to push the rating up purely to push the rating up can make an experience uncomfortable or unenjoyable. However with BG3 it seems like it is an intended part of the art, Mindflayers being a big reason because anything with them is meant to be concerning at the best with mind control and enslavement, and overwhelmingly gruesome at birthing out of people and intellect devourers puppeting corpses. The sex also seems purposeful for the characters presented, none of them seem chaste and sex is a common instinct, especially in situations like the ones these characters are in. The only issue with sex is that for a player it may be a little too forefront and so it would generally be better for there to be some more direct flirting on the part of the player before every character wants a piece of them.
I'd agree with you IF it felt like the mature elements were forced, but to me they don't, they feel like a part of the intended experience. So I think the game should be comfortable with a M rating.