When? How?
First they'd have to convince the adults theythe Yeerks are real. Then they'd have to convince the adults they're friendly. then they'd have to convince the adults to give up their lives to be hosts.
How could THIRTEEN year olds do that? THIR-****ing-TEEN.
No adult will EVER believe a THIRTEEN year old.
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When would they have time to do this? With school, parents, and all of the loyalist Yeerks infiltrating the cities. When would they have time to fully explain and convince adults in other parts of the world?
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Visser 3 would never allow a Visser below his rank be isolated with him. Don't you understand? Visser 3 is paranoid about everyone around him because they could be an Andalite. Unless they obey his command explicitly, they die.
No Visser would be able to say, "Hey, Vee-three, would you like to come into the next room and have a chat, maybe have a cold one?"
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How would they replace his guards with YPMs? he has so many and they're killed left and right for being incompetent.