Maybe there's a yeerk whose job is simply to control Alloran while Visser Three feeds? Like in an empty, solid walled room, a yeerk controls him. Everything is locked down, so if the yeerk tried to escape with Alloran's body, it would simply starve from lack Kandrona. When Visser Three is done feeding, he's brought in, and swaps hosts with the yeerk guarding the body. Alloran can't escape or hurt himself, Visser Three has time to feed, and the temporary Yeerk wouldn't try to escape with the host because it knows it will starve.
The only issue would be knowing parts of Visser Three's motives, but possibly that yeerk is kept in isolation or Visser Three has it killed each time. We know he would do such things.