The fact she has been very honest about the whole thing, is an indication of her being a good girl.
It sounds like you don't fully trust her, which is understandable. Some people would argue 'once a cheater always a cheater'.
She has no intensions of cheating, otherwise she wouldn't be that voluntarily honest about the past.

In your case I would definitely sit her down for a proper talk and clearly state your boundaries. You have to do this now, because it's still early in the relationship. Tell her you are concerned about this project because of her past (she knows that already, you show her you are honest about it), but going to trust her with this film project guy. Tho you also expect her to be professional at any given time. Definitely let her know that if she cheats, that will be the end of your relationship. This has to be done very firm. This will make her less likely to cheat if she knows it's over. Say this only once but in the most convincing way you can. If you repeat this to her, even occationally, that would be a sign of insecurity.

Read the post twice, first read figured in these situations it's better not to put thoughts of cheating into a guy's head. Second read this stood out:
I believe past is the past, but you should look at someone's past for reference on how they will act in the future. If they did wrong in the past, and have not shown change, then the past will repeat itself.

The above quote highlights a concern and red flag. When asked about whether she might cheat, she didn't say because she's changed and realized it was wrong to cheat, that she doesn't think about other guys, that she loves you, but that she may not because things aren't as bad as they were with her last bf. She's a cheater IMO. She has basically said that if things get to how they were in her last relationship, she would or could cheat. She is not the type to break up with you if things get bad, she has admitted to being able to go with her emotions when the fighting starts.

If she thinks like that after cheating it's a problem. She hasn't changed, the only difference is your fights are not to that level that she will cheat now. She told you :"Things are ok now so I'm faithful. But if things go bad, or kinda bad, I won't break up with you...I'll allow myself to cheat."

Why tell her you're very happy with her if you're not? Maybe she's a good lay and you guys have a lot of inside jokes established, but here you are. It's been one month, there's a roadblock, and you're not happy about it.

You know you can't go forward with this relationship unless she drops the project. But this is where it's tricky: you can't tell her to drop it, because it isn't your business yet. Just like she doesn't deserve proof of commitment when none of her behavior, other than fucking you, merits it.

So, if she sets this project aside, allow her to come to the conclusion ON HER OWN. Any ultimatum you give a woman will make her want to do that thing even more.

I agree, I don't think she's intending to cheat. I haven't explicitly said that it would be over when she cheats, but I think she knows. She knows how I handled my last breakup (fair but firm). And when I talked to her it seemed like that's the premise for her as well. I don't come off as a weakling so I don't think she thinks I'd accept her cheating.

To this I have to say, yes I see where you're coming from. But since I couldn't write down every conversation we had, I did not mention that right in the very early stages we had a lot of talks about previous relationships and she's told me that she deeply regrets cheating on her ex and she's looking for a healthy relationship and now knows better than to cheat, after experiencing and realizing her loss. Then again, her next relationship ended with her cheating, too. But she says she wasn't serious about the last one.
In both cases though, I believe she cheated because the relationship was over for her and that was her (weak) way of dealing with it and getting out.
But you're right, the bit, where she thought about the correlation of her past and this situation and when she said it was "maybe a little bit similar" is still a big warning sign for me.

Only after that talk she told me that about her SPAM session and the red flag for me was that it happened after our little fight.