so one question.
if open war is so much more attractive than underground invasion, how come the Council of 13 was never bright enough to notice? I mean, given they are parasites and stuff, but they are also brutal and logical.
However, I do believe that the underground invasion favored the Animorphs also because they were able to do infiltrating of their own. The Animorphs did more than enough by themselves.
The simple answer to that is Visser One. Edriss orchestrated the entire invasion on Earth and it was her decision to make it a silent, underground invasion. She was the expert, and the Council trusted her to make the decision that was in favor of a Yeerk success. After Visser Three took charge he did repeatedly try to change it to an open war, but the council did not trust him as much as they trusted Visser One.
EDIT: It should also be noted that Edriss did not choose silent warfare because it would best aid the Yeerks. Initially, perhaps it was a strong war tactic, but it became obvious later on that open war would be more beneficial. However, she continued to petition for infiltration due to her own personal interests, not the interests of the Yeerk Empire.
Yeah, agreed on the hellhole thing. Regardless of Edriss' motivation, I still believe a silent invasion to be the right choice for the Yeerks in taking Earth, given their situation. There was a shortage of hosts in the empire, which humanity could help alleviate. Their resources appear to be spread fairly thin. How many Hork-Bajir and Taxxons would the empire lose in an open war? Sure, they can burn our cities from orbit, but they still have to capture humans alive, and that requires putting their people in harm's way. The resources the Yeerks would need to sustain an open war with humanity might not be available due to the war with the Andalites.
Then you've got to consider the payoff. True, a few million hosts would be huge to the empire, and help them sustain their power, but it doesn't give them any huge advantage over the Andalites. In open war, a billion human hosts seems like an exceptionally generous estimate. If the Yeerks manage to take all six-billion-plus humans, they become unstoppable; even the Andalites would have no chance. If they attempted to take Earth by force, I think they'd need a much higher concentration of troops here, possibly bringing the brunt of the Andalite military down on this invasion and effectively crushing the empire.
She may have been trying to save her children, but in my opinion, Edriss' decision to invade Earth silently would have saved the empire as well, without the interference of the Animorphs.