Leareth tells the Taxxons who are voluntary controllers, in private Mindspeech, that Yeerks who took only voluntary hosts are likely to be pardoned by the Andalites, and that on Earth many human Controllers requested to keep their Yeerks and this was allowed and actively supported. It's very important to him, personally, that this war not contain any more death, and loss of more intangible things than lives, than it needs to. Anyway, if they want to keep their Yeerks, wanting to keep their Yeerks is allowed. If they're worried that the Andalites garrisoning the planet are going to feel differently, well, he's happy to take them to Earth, where things are a bit less tense.
Anyway, if they still want to try the Velgarth compulsions, he'll attempt it first on one of the Taxxons who wasn't a voluntary Controller and he can see how well it even works. First a simple one to keep them from eating anything while he tries something more subtle, then - hmm, he wants something on the level of goals-and-motivations more than actions. Really what he wants is a Mindhealing redirect, something analogous to a calming loop except instead it's an away-from-hunger loop, with a conditional firmer compulsion the Taxxon can trigger on themselves for a set time period if they're close to losing control. It takes him a while of fiddling to get as close as he can to replicating the thing he wants.
And then they should probably be put in a room alone with some food, minus Yeerk, and Leareth will remove the unsubtle action-compulsion - from outside the room - and they can test if they're capable of using willpower to not eat the food.