Yeah, if you're going to keep up with this new character for the players' POV, I would make that entire thing optional, because otherwise you are going to get A LOT of anti-NTR hate-mail coming into the thread. I'm sort of middle-of-the-road on NTR stuff, it's hit-or-miss for me a lot of times, but in this case I'm not a fan so far. I pretty much agree with everything Dragon X Hunter said.
In the old version, it felt the MC was Tamara, and we just get to see other characters, because that's normal in most works of fiction. That meant that we the player were identifying with Tamara and enjoying the kink of exhibitionism. Putting us in the roll someone watching her completely removes us from being in the position of experiencing being exposed and flips us to being a voyeur. It's a sort of reciprocal but exactly opposite kink. I fear you may lose a lot of your audience this way.
Also, yeah, if we are going to be following her around when she gets fucked, it would be nice to at least get to participate. My only thing against that is that with it the way it is now, this unseen character could be a fellow girl who enjoys watching Tamara, and obviously having us pull our junk out and fuck her would force the character to be male (or trans, or a futa I guess.) So if you decide to go the route of letting the player have sex with Tamara I would again recommend making that optional, so that any female fans you may have won't feel suddenly excluded by forced to be a male character.
With a couple of simple choices and switches, I think you could keep the game linear with only minor calls to skip certain lines/images. All the same events occur except there would be three optional modes, like so:
1. Player chooses to skip this new character entirely, this turns off all dialogue referencing the character
2. Player chooses to be her boyfriend/fuck-buddy, this turns on additional moments where player can have sex with Tamara
(If all we ever see of this character is a penis, the player could choose to be male, trans-female, or futa with zero extra work)
3. Player chooses to be a female friend who just likes watching Tamara do her thing, it plays like it is now, but it's not NTR
(note that some people will still say it is, but if it's two girls that are not in a relationship, that's a hard argument to sell)
EDIT: Seeing your comment on this issue, I see your point about ethnicity. That is always a hurdle. You don't want to spend a lot of time making alternate versions of scenes to avoid making people feel out of place. This is an odd sort of character that's a POV character but still not really the "MC." Tamara is still basically the main character, and the less said about who this POV character is the better. Maybe it's just Tamara's roommate at the apartment? At any rate, while I would never advise this in most AVN's, I think in this particular case you could get away with just not showing the penis at all. Just let Tamara say we're fucking her and show us a close up of her. You do it that way, this person could be white, black, Asian, anything and could be a man, a trans-woman, a futa, or even a girl with a strap on with no extra images to make for any variant. Have Tamara refer to the person by name as much as possible, so you don't need to worry about alternate text for pronouns. In fact, this way, you don't even need to make the player declare who they are beyond the name, just point out to them that they could be any of those things they want to imagine.
