Even if you trained the AI yourself from scratch you still can't be confident you know what the AI is going to say under any given circumstance. LLMs have an inherent unpredictability to them. That's part of their purpose, they're not databases or search engines.

if I were to download a pre-trained model from what I thought was a reputable source, but was man-in-the middled and provided with a maliciously trained model

This is a risk for anything you download off the Internet, even source code could be MITMed to give you something with malicious stuff embedded in it. And no, I don't believe you'd read and comprehend every line of it before you compile and run it. You need to verify checksums

As I said above, the real security comes from the code that's running the LLM model. If someone wanted to "listen in" on what you say to the AI, they'd need to compromise that code to have it send your inputs to them. The model itself can't do that. If someone wanted to have the model delete data or mess with your machine, it would be the execution framework of the model that's doing that, not the model itself. And so forth.

You can probably come up with edge cases that are more difficult to secure, such as a troubleshooting AI whose literal purpose is messing with your system's settings and whatnot, but that's why I said "99% of the way there" in my original comment. There's always edge cases.