So, the Mertil and Gafinilan thing?
Where to begin.
Andalite logic is used to explain why neither one would make a decent enough host.
One would argue maybe V3 has to contend with his host's likely ableism and that might color his own opinion and rationale, but ugh... UGH... Why can't the book just be clear on the point that V3 wants TO KILL both of these dudes because he wants to remain the only Andalite controller? I hate that this is never outright discussed in the series, and that certain books seem to confirm that this is the case (if only because the decisions V3 makes in those narratives tend to be so inexplicably dumb outside of that consideration), while others flat out work against it (side-eye at Taylor).
I wanted so badly for this possible internal conflict to be a revisited thing. We have Eva/V1's loyalties picked apart and her situational investment is laid bare as a primary motivating factor for her character's actions. I've always wanted to consider a V3 who tows the party line when convenient, but would actually rue the day that the Yeerks directly engage the Andalite homeworld because he feels he's the only slug around deserving of an Andalite host. I mean, there's so much rumination from nearly every character's perspective who comes into contact with V3 that the mere fact of his Andalite host body makes him unique and powerful compared to pretty much every other Yeerk... Other than that, he's just sadistic and crabby. Not too special.
But yeah... giving up on the tactical advantage of possible military secrets re: Mertil and Gafinilan... SO DUMB, V3. SO DUMB! You could have dumped a trusted lieutenant into Mertil's brain, allowed dude to be rescued, and then known everything you would have needed to know about the resistance on Earth to just bang this invasion out. FAIL!