All we have is an achievement that unlocks
Mohandar's portrait. Not an achievement that unlocks Mohandar. The portrait does not have to have ANY use in the campaign; as long as they've made it, they can put it to use as an unlockable in the MP.
I'm 100% with mr. peasant.
^this. We're bound to get face time with Artanis and Selendis at the very least, and that's already on top of 2-3 missions that revolve around Zeratul. There's only
so many characters you can service well within the scope of a single campaign.
Which isn't to say I'm absolutely certain they're not doing it; or even that it can't be done right, maybe it can. But I wouldn't bet money on it.