It's 50/50 whether Ava cheated or not. I can recall times at parties I've snuck off to a private room with a girl to talk in peace and quiet. The possibility for sex was always in the back of my male mind, but it wasn't always the same from the female perspective. Unfortunately for me, that was the case more often than not.
There are literally a thousand scenarios with differing levels of guilt for Ava. If she went upstairs with a mindset to fuck the shit out of Bryce and followed through, I toss the bitch out the car the first chance I get. Even if her inhibitions were lowered by drugs and alcohol and she pushed Bryce away at the last moment, it still would be hard for me to forgive her and she'd have to do a lot of persuading.
However, at the other end of the spectrum of possibilities, what about some kind of sabotage scenario orchestrated by a jealous culprit? What if Ava went upstairs to just talk about old times with Bryce before he departs. Natasha enters the room at some point. She had been trying to force Ava on Bryce all night, as well as pushing drugs and booze at her. Maybe Natasha has a thing for the mc we don't know about. She offers Ava a beverage laced with something. Ava eventually passes out, leaving her vulnerable to having her clothes removed and staging a scene to enrage the mc. As you point out, all we see the next morning is Ava and Bryce asleep on opposite sides of the bed, Bryce turned away from her with his underwear on. It wasn't the typical after sex scene we see in these porn games.
If I recall, the game mechanics allowed for positive and negative actions in the MC's choices. For example, there were some social media posts where you could respond kindly or rudely. Also, at the store you could help the mother with her child in a positive way or not. Maybe this is the way the mechanics will play out in general, as well as in interactions with Ava. Will you make choices that push the mc towards pessimism, depression, and reckless behavior? Will you continue to be the guy who stormed into someone else's house and beat Bryce unconscious without stopping for a second to listen to anybody? On the other hand, will you make choices that push the mc towards a more positive outlook and constructive behavior? If you take the petty, spiteful route with Ava, you may never pull yourself out of your funk and may never learn the truth about what happened that night. On the other hand, if the mc makes more rational choices with her, at least giving her a chance to hear her out, you finally learn the truth.
Okay I promised a full reply and here it is.
Add my main point about it being manipulative towards the players with some life experience. After discussing the scenario with several irl coworkers, they and I believe that even if she did back out before anything happened that her excuse of what did happen being because of the drugs and booze is bogus. Even while very drunk most people still have their base morals that they won't cross. A terrible example but if you wouldn't kill a dog sober you most likely aren't going to while drunk. That takes out the possible excuse of "I was drunk".
Now you do bring an interesting point, many times guys will try to get girls away from a party to "just chat" and not be able to follow through with their "clever" scheme. I too have struck out with this tactic. However, I have yet to meet someone who fails and still wakes up to the girl naked and in their bed. Of course, "something" could have happened that ruined her shirt, and good guy Bryce is washing it for her to wear home. But then why not offer her one of his? Why sleep in the same bed? Why was he sleeping in his underwear if nothing happened? Do you know any woman that would sleep almost nude to a man that was just a platonic friend?
Then comes the texts from an unknown number. Now, these could be as you suggested from someone who wants to get MC to themselves. I will allow that possibility but I will say that it is suspect as this event seems to happened some time ago and even if MC became a loner, someone who put all of this together to make them single, would not have just waited for months if not years before trying something. If nothing happened, I think the texts came from Bryce's phone but were sent by Ava to explain why she wasn't coming home that night but she also sent a picture with a little too much information in it. Of course, we as players were meant to think these were from Bryce bragging about what happened but that obviously wasn't the case as he would have been ready for the fight that was coming. So those still remain a mystery.
Your theory about the concept of the game is sound but it still relies on several things. First, forgiveness and not just the MC forgiving her but she forgiving him. If nothing did happen then she would have to let go of her ex beating the ever-loving shit (at least in my playthrough) of one of her good friends. That is its own can of worms and I am not going there right now. Back on the other side, MC would still have to forgive her and this is where things start to become too much. Either she cheated and has the gall to ask for forgiveness to ease her own pain and guilt over the act or she stopped it but still did everything that lead up to that situation. Remember the MC clearly said "don't overdo it" and well no arguing that she did that at least. Next, that has to happen though is a conversation where she tells "her side" of what happened. It is going to be Annie that talks us into that conversation as we know MC doesn't want to hear it but even after hearing it, if she was "soooooo" drunk that she got in over her head, how would she remember what happened?
That last point brings us back to the biggest convenience here. More than likely the picture that Annie saw just before leaving our house was Ava. So even if she didn't plan to pick her up, which let's be real, Ava is going to New York as well and even the MC called that one out, so yeah she sat this whole thing up. But if she didn't, then she at least knew what Ava looked like and still allowed her in the car. Now I am all for the idea of friends helping others out of dark places, but can anyone say that being trapped in a car with your ex sounds like it would lead to anything but constant arguing or angry silence?
All in all, I actually liked this game. The cinematic earned this thing so many brownie points that I will play the next update even if I end up hating it. But the base idea of manipulating us as players to feel and act about a character only to pull the curtain back and show that we too were set up to go this way is wrong. We did not need to spend the party with Ava to get the hint. The texts and walking in on her naked would have been enough. But to go this route and then explain every single bad decision away, well that is something that I have no kind words to speak about. But to clarify, I am not hating on the dev. They are obviously talented and I expect they will do very well and that is why I am so upset by the possibility of this paint by numbers "nothing happened" story. I say possibility as if we are all wrong I will happily eat my words and apologize. But if not, then this looks like a project that while visually stunning will be a story as fresh as last week's bread and that is just sad.
Oh and to answer your question of how I would proceed with Ava, she "slipped" off the roof of the hotel, tragic accident.