...worst case scenario, the Powers can feed a "here's how to make the Yeerk wrapped your brain be somewhere else / disappear / ... just by thinking about it" directly to the new Controller's brain.
Maybe they could - but would they? They could also have fed Nita a "make the nasty aliens just go away" spell during the fight at the Crossings, and given her the power to use it (maybe tap one of the many worldgates up there); instead, they let her stumble through with the aid of a non-wizard with a... hairdryer? It's been a while, but I think it was a hairdryer.
The books are full of examples of things the Powers could just create a spell to fix. Even if we exclude War as exceptional times, you could resolve every plot with a lot less pain and risk and loss by just giving Nita and Kit a shortcut spell. "Make my parents not notice my absence" would be a neat one for Deep, and well within their normal capabilities.
But they don't. Because doing it the easy way is not the point - in fact, it's Someone Else's point. And - a certain incidents involving a star-forged spear aside - the Powers don't hand out reset buttons when things go bad.
So how would a Controller Wizard story go? Depends how they got in that situation. If it came about because of the wizard's choices - such as staying behind to let others escape, knowing the risk - then I think they'd need to solve it themselves. The Speech is a persuasive medium, especially when you're in direct mind-to-mind contact, or failing that there's the brief interlude when the yeerk has to be back in the pool.
If they didn't have a choice - say they were infested in their sleep, before they even knew the yeerks were there (it's not mentioned in the Manual because their planet isn't aware of it yet) - then you have more options. The most low-key would probably be to give them a mental connection to another wizard - think A Wizard Alone. Your Controller's friends and allies making a combined mental and physical effort to free them is much more plausible than "oh dear, that's a problem, here's your fix-it spell", I think.
Of course, any of these scenarios leaves a yeerk with at least temporary access to wizardry - but probably no Manual access, of any kind. The damage they can do would be limited by what wizardry their host can remember, rather than just anything in the book. Unless they get hold of a Planetary (gulp!), we're probably not in end-of-the-world territory.