Since the Yeerk's physical body no longer exists, it wouldn't die from kandrona starvation after 3 days. A Controller who morphed falcon in #52 actually claimed they wanted to go nothlit.
Actually, are you sure that was a controller? Ax assumed it was a controller, but it could also have been just a morphed Yeerk. That would have made more sense if the Yeerk intended to trap himself, since the Yeerk probably wouldn't have known whether he'd die after three days or not, and might not want to risk it. And besides, it seems like it would have been really wasteful to trap a perfectly nice host body in morph with you rather than just being a Yeerk trapped as a falcon (not to mention that the Yeerk would have to listen to their host's whining for the rest of time).
Of course, we don't actually even know whether the Yeerk even intended to trap himself at all. There's at least a chance that it was just a ploy to get Ax to let him go. Let's not forget that Yeerks are some of the most talented actors in the universe.
Anyway, going back to the subject at hand, I agree that a morphed controller's Yeerk in Z-space would be able to live indefinitely. As is shown in the series, morphed human bodies in Z-space don't need oxygen (unless they're disturbed by passing ships), and Tobias's human body would have been locked in Z-space long enough that it should have started needing things like food and water when he was trapped in hawk form. If he could survive without those things, then it's conceivable that a Yeerk could survive without Kandrona.
