In your example, there are missing details. Who owns the output? The way you've described it, that would typically mean that the guitarist is creating a "work for hire" so the ownership transfers to you, but that's a contract detail that would need to be resolved.
For whoever owns the output also owns the liability of the output. You yourself might separately be able to pursue a claim against the guitarist for breech of contract. In the process of that, it might get discovered that you deliberately instructed the guitarist to copy the work, or they copied despite your instructions not to.
But that doesn't change the fact that the final work is infringing. It just allows you to pursue damages that could potentially offset any damages you're liable for from the infringement.
But this also isn't exactly the same situation as OpenAI. OpenAI isn't an individual creator working on contact for you. Even if their ToS ultimately assigns copyright of output to you, there is a matter of scale involved that I think changes things. It's one thing if your guitarist damages you by doing shoddy work, it's another of the guitarist systematizes and scales their shoddy work to damage large numbers of people. Perhaps that would then become a class action issue.