Say what you will about the Auxiliaries being a desperate measure that the Animorphs had to take, the risk of one of them being a controller really should have outweighed the benefit they gave, and the Animorphs were darned lucky that that wasn't the case.
Just imagine that for a second, if the Animorphs had been wrong and one of the Auxiliaries was a controller. An Auxiliary as a controller would have been able to pass completely undetected, because the Animorphs didn't know any of them well enough to be able to tell if one was acting funny, and the other Auxiliaries didn't know enough about the behavior of controllers to have been able to know what was wrong, either. And that one controller would be trusted enough by the other Auxiliaries to very easily be able to start making them into controllers without anybody catching on. Once they were all taken, with that many people who were that close to the Animorphs, it's entirely possible that they would have had the strength of numbers (and the element of surprise) needed to take on the Animorphs and win.
Game over.
There's a difference between being severely burnt and being disabled.
One of Taylor's arms was replaced, which leads me to assume that that arm was either lost in the fire or at least severely damaged. I'd call that being disabled.