1) Why didn't Elfangor aquire one of the pre-Animorphs's DNA, change into them, and heal his wound. That way he could been the mentor figure of the team. He would also have a relationship with his son, Tobias.
Several possible reasons he didn't just morph something battle-capable to self-heal and self-defend (maybe after Ellimist yanked him back into combat, he wasn't given a chance to acquire anything?)... and as for morphing a human, he'd scare the kids half to death in the process, they'd panic and run, and all of them would be killed or infested while they ran away. By dying, Elfangor served as a distraction to V3 and his minions.
2) Why do the Animorphs only use Earth-based animals instead of the aliens that they have aquired? Here is an example: Hork-Bajir. They are much better combat morphs than Earth animals, have pretty fast healing properties, and since the Yeerks have plenty of them, perfect for sneaking into a base and getting some info. Yet they stick to lions, tigers, and bears.
Maybe Hork-Bajir all look the same to us, but to other Hork-Bajir, they have distinguishing features. They can tell themselves apart from each other... and it would give the animorphs away, regardless. On the other hand, there's not much to keep the animorphs from killing each other by accident, since they don't have much experience telling Hork-Bajir apart, and they don't talk to their enemies much. Also... author license. XD
3) What point did the Ellimist and Crayak serve? I mean, I know its science-fiction law to have god-like beings, but what was the point. Sure the Ellimist gave Tobias his morphing powers back, but thats about it. Maybe I missed something.
Deus ex machina. Diabolus ex machina. Sudden plot twists. Wild cards. Remember, this is an extremely episodic series, with very few sub-plots and story arcs occurring over multiple books. The series needed Ellimist and Crayak just to provide fresh material for episodes... the same way the Chee were necessary.
4) Why is Cassie so useless? She's not that smart, not that good a fighter, and constantly complains about who the animorphs have to do to stop the Yeerks. I mean, if I was part of a guerrila fighting force trying to stop the entire enslavement of the human race by a bunch of slugs, I would probably do anything in my power to stop them. Also one of her parents works at a zoo, so she virtually is the one who has the key to tons of new morphs.
She is an audience avatar for the sake of tree-hugging readers. She's the obligatory conscience. If she isn't there, it's hard for this series to pass morally/ethically as "oriented to be read by people of impressionable ages". Also, Rachel is only "girly" in the stereotypical mall-hopping, clothes-obsessed ways. She's not deeply feminine... and I figure that K.A. wanted at least one "psychologically stereotypically feminine" main character, so that female readers wouldn't feel left out. The end result is kinda'... derpy...? But then, the characters have certainly shifted in their development, over the course of 60-some volumes.
5) Why does Visser Three know so little about Earth and its animals? I mean the guy has tons of Human Controllers with memories of those animals, you think he would use that information to his advantage.
Gleet bio-filter. Radar-equipped hunter-killers in the caves around the Yeerk Pool. An increase in security at The Gardens. I think, considering V3 also has to micro-manage his own personal invasion of Earth, he's done fairly decently at taking steps to hinder the Animorphs... and his one major act of stupidity is that he failed to assume there were humans involved in the "Andalite Bandits". Who but an Earthling would use Earth animals so avidly in battle? A true Andalite WOULD be able to tell Hork-Bajir apart from each other, having been around them more historically.
6) Why is there some many similarities to Power Rangers? Think about it for a second: five teenagers are given the ability to morph using the power of animals by a wise alien who is a famed warhero to battle, in secret, an alien warlord commanding an army of monsters. Along the way the group of five get a sixth member to aid them. Think about it.
PR and Animorphs are both generic 5-or-6-man bands. They show up all over fiction. Transforming for battle also shows up all over fiction. This stuff saturates the media directed at kids and teens. The resemblance isn't just coincidental or deconstructive... it's culturally fundamental. The formula shows up in way more places than just those two outlets. Animorphs is full of literary tropes, and this similarity is by no means surprising. The similarity in the names is more or less a point of inside humour, but again... not surprising.