The standard spiel, as told to people who have normal problems in this genre, starts by saying that alignment is a shortcut. Just because Law is an ideal to aspire to doesn't mean it actually does anything in negotiations other than tell you a little bit about how the other person thinks. (Not even all that much! Someone can project "Lawful" and have that name mean all sorts of specific things, after all, and some of them are Razmir.) If you're working cross-alignment you can't instantly trust your counterparty on questions like "a fair split of profits," but you can't always do that anyway. You're doing without the shortcut and that's fine.
The good specialists, such as the ones here at Swift 22.214.171.124, won't give Cam the standard spiel. They can avoid assuming most people are Lawful Neutral and like it. (He might still notice the Lawful Neutral baseline hanging around in the background.) For Chaotic Good people, it sometimes helps to think of this whole "Lawful" construct as less a fundamental force of the universe and more just a bunch of people doing their own thing, except that it's the same thing and they're doing it because it scales well.
...although some people can get very annoying about how everyone else should be Lawful. Other people might make it a pillar of their personal identity, and it sounds like Carissa might be doing that one. Not to armchair speculate, but Carissa might have been thinking of Law as the source of every good thing because in Cheliax that was basically true. That particular issue tends to get less pronounced with exposure to. Like. The universe.
But yes! People in Carissa's position thinking Cams are fundamentally untrustworthy is a common problem! The most basic thing that needs doing from Cam's side is showing the Lawful person that it is actually possible to form expectations around his future actions at all. Convincing her of specific choices Cam would or wouldn't (or could or couldn't) make-- it's hard for that to be reassuring without a shared foundation of "it matters to this person whether things are logical." Which Lawful types sometimes conflate with alignment.
When any miscommunication can sound like something unexpected and anything unexpected can pattern-match to "this person is Fundamentally Unpredictable," the usual solution is to go through intermediaries. You can talk to your lawyers talk to my lawyers talk to me. A trained mediator will respond to things like "is pointing out you could already communicate with Lastwall a threat" with either "no" or "let me confer with my principal" and then "no." Or "yes, and here's what it was trying to accomplish and why it seemed like the best option," but it's usually no.
They can work out the communication problems easily enough, especially since Iomedae already handled some unknown amount of that. Assuming the process is taken care of, what does Cam substantively want from an agreement? Ability to conjure on his own recognizance, ability to directly oppose Cheliax, a promise that Carissa won't banish him?