I know this is a seven-year-old question at this point, but I actually have an alternate theory about the why-do-Yeerks-die-when-stepped-on thing. Which is that
they don't.We have two examples, in the series, of Yeerks 'dying' when they get stepped on. In #12, when Rachel steps on the Yeerk formerly infesting Jeremy Jason McCole, and in #45, when Marco steps on Edriss. For Rachel's part, I think she says something like, <I guess the Yeerk didn't make it very far.> Marco's thoughts in #45 are similar. They both just more-or-less assume that stepping on something would naturally kill that thing, and they move on (in both cases, they don't really have time to stop and check). Plus, since Yeerks are clever, my bet is that most Yeerks would 'play dead' for at least a few seconds after something like that happened, playing up that assumption that a human would make, that stepping on something kills it.
Heck, both Yeerks could have survived a pretty long time after that (assuming Edriss had the good sense to keep a low profile post-war). JJM's Yeerk could have been rescued by one of the other controllers nearby, and Edriss needed only slip back into the pool when no-one was looking.
When Visser Three has to leave Alloran in order to feed on Kandrona rays, how is Alloran prevented from morphing something small and escaping?
I believe the series states that Alloran has to be rendered unconscious every time Visser Three feeds.
The Mutation: How did Visser Three acquire the creature that seems to be made of fire?
Very quickly.