So...
I'm ridiculously late, I know, but I am finally in a position to speak to this thread because I finally finally finally bought the first two seasons of DS9 (yaaaaaaay!), and I watched "Emissary" last night. So, my answer to the OP is...
No. Alaimo isn't the only actor to nail performance in "Emissary." Alaimo did nail it, of course, but I think Avery Brooks did as well.
Yeah, yeah - I'm not saying Brooks is perfect every single episode, and in fact, I can think of episodes in which he most definitely didn't nail it. I generally like Brooks' acting style, but jeez, he can be a bit or more than a bit overwrought, and I guess I pretty much assumed he had been in "Emissary," too - there was so much potential there for melodrama and overacting.
But there wasn't any. Not that I saw anyway. The flashback scene where Jennifer dies? Just spot-on acting, no overacting - I mean, jeez, the guy had just discovered his dead wife. The scenes with Picard? Wouldn't change a thing. The conflicts with Kira? Ditto.
He nailed it. And he had a much more difficult role than Alaimo, too - just because so much of the show revolved around him. Don't get me wrong, I don't want to take a thing away from Alaimo, who took a very minor role and turned it into something extraordinary. I mean, you can't take your eyes off him when he's on screen. But I'd say Brooks nailed it, too.