<You will tell me everything about the so-called peace movement,> Visser Three told Aftran, blasting his thought-speak loud enough for everyone to hear. <Then I’ll have to use my imagination and come up with a nice, long, painful way for you to die.>
In #43 we learn that Visser Three has figured out when the Yeerk Peace Movement meet at the Yeerk Pool, so presumably he has some suspicions on who might be involved. According to the Yeerk Illim, there were "nearly a hundred" members at the time.
1. Why didn't Visser Three use this technology on subordinate Yeerks he suspected of being in the Peace Movement?
Why does he have to interrogate them in a "human" fashion?
Perhaps the Council has strict control of this technology and even Vissers aren't allowed to use it? In that case, Visser Three could take the hosts of Yeerk suspects while they were in the pool, and then stick a loyal Yeerk in there to access all the memories of the host. Typically, there'd be plenty of "incriminating" memories in the host's mind. For example, Illim had formed an alliance with his host Mr. Tidwell, and they worked together in the Peace Movement. Therefore, if Visser Three put another Yeerk in Tidwell's head he could extract a great deal of evidence and clues.
2. Why didn't Visser Three just crush the Peace Movement by accessing all the memories of their hosts?
For example, he seems to have figured out when their feeding meetings were. Why not just conduct massive sweep checks of hosts at that time?
I don't know if Aftran had any hosts after Karen, though. But what about the other Peace Movement members?
Perhaps this was a rare case, and most Peace Movement Yeerks held their activities secret, even from their hosts? In this case, they'd have to restrict their communications to when they were in the pool. This hypothesis is corroborated by the fact that they intentionally synced up their feeding schedule to feed at the same time. However, if they have to hide their activities even from their hosts (who can see everything they do), how could they do anything useful? Poolside propaganda?
See also: Did any Yeerks other than Illim know who the Animorphs were?