Its slightly harder to believe that someone could record the complete narration of such a film without getting some idea of what it was about - or at least getting suspicious. Nor does it pass the plausibility test that the makers would go to the time, expense or legal risk* of large-scale manipulation when there are plenty of real life Troy McLures out there would will read out whatever the hell they were handed if they needed money or lizards.
You seem to think she watch watching the film and narrating at the same time. But I suspect it's pretty common to say "here's a bunch of things we want to record you saying and here's the context", she might not have seen any clips nor even expected to watch the final product. She probably thought she knew what it was about, they probably gave her the context and backstory for every line, but if it was a lie and she trusted them it would be pretty trivial for them to get her to say incriminating stuff.
They could probably even get her to say something nice about geocentrism if they framed it right, ie "Ok, we're talking about the origins of astronomy when they first got the idea of planetary bodies, so we want you to read 'The geocentric model of the universe answered many questions that had baffled philosophers for ages'"
That sounds like it could be a credible statement, but can obviously be used in a very different context.
As for the "Troy McLures", they certainly exist, but they don't have credibility. The subset of actors who do narrations and have decent geek cred isn't huge. And it doesn't take a genius to realize that appearing in one of those things would cost them far more than the measly amount of money they'd make.
Like lots of former Trek actors Mulgrew still does a lot of genre work, you think she's jeopardize her career so for a tiny paycheck from a geocentrist documentary? Her claim that she was deceived is far more credible than the idea she was willing to destroy her reputation for this documentary.