My problem wasn't so much with the smuggling of weapons or the ability of a slave to lead an uprising. Or even putting a madman on the throne. We see lots of examples of that today.
My problem came from a Trek context. Shinzon said that the only thing the Romulans hated more than a reman was a human. So, the believability, for me, is hamstrung by the fact that Romulans conspired to put a human who acted like a Reman on the throne.
I mean, are we even concerned about having any sort of dedicated claim to the office? Could any human just waltz in, promise dreams of galactic bloodshed and be placed on the throne by some bunch of power mongering half-wits? It was almost like the plotline of "Head of State." I half expected the Reman Viceroy to be played by Bernie Mac.
Maybe just a little bit of explanation as to why the Romulans would do this might have helped. basically, it was just, "hey, we're Romulans. We plot. That's what we do."
Then there's the whole telepathic rape thing. I don't know about you guys, but if I was Shizon, the only reason I would have been contacting Deanna would be to ask if she could give me the phone number to call up Ezri Dax.
"Oh, I'm going to conquer the galaxy. But first, I want to tap this betazoid chick for a while. And then, I want to do a gene transplant on this human guy. And then I want to insult Dina Meyer and threaten the very people who helped put me in power. And then I'll get around to destroying the galaxy. Oh, of course I mean 'conquer'! That's what I meant! What was I saying, again? Oh, you've made me lose my place! Perhaps later you and I will have a proper discussion about distracting me, filthy romulan!"
Strictly Speaking
"When you meet the Buddha, kill the Buddha."