You make a very good argument for that theory.
It’s very likely the Zovaal we see is just a projection of the Runecaver who is in fact Zovaal himself. It’s also never been fully explained how the Jailor broke his bonds too or even if Sylvanas with his boon had the ability or knowledge to do so. You would of thought whomever put him there in the first place would have created safeguards to ensure he or his allies wouldn’t be able to.
What’s even more compelling is that we might have already been forewarned in some way of this type of deception and treachery.
If you played through one of the side quest chains in Revendreth there is one where we encounter a chained up Venthyr called Sabina in the Ember Ward.
Where she two has (or at least claims to have) lost recollection of her past events that led to her imprisonment. Although initially she pretends to play the victim, we later discover in our attempts to find a means to free her, that she herself murdered her companions and then proceeds to turn on us when freed where her darker side resurfaces as she finally succumbs to her torments.
It’s a bit clutching at straws but, I wouldn’t be surprised if a similar series of events were to play out involving the Runecarver as well.
Remember too that Ve’nari (another dubious character) did give us the following word of caution as well, when accepting “The Weak Link” quest.
In the quest text she says,
“This ought to be enough to break the bonds that hold your newfound ally.
<Ve’nari pauses for a moment.>
Before you do so, I offer a word of warning.
No creature imprisoned in that place should be trusted. Tread carefully, mortal.
After all, it would be a shame to lose so valuable a business partner.”