differentiator is beta software; it might mess up. If it does, I'll try to make it right, but I can only promise this if the losses are less than M$500 and you contact me promptly. (You can also get in touch in the Manifold Discord server.)
Manifold Markets doesn't yet support scoped permissions, so your API key gives access to your whole account. Your API key will be stored on differentiator's server; you can reset it in your account settings any time (though this will break any actions you've asked differentiator to take in the future, like cancelling limit orders).
Currently, differentiator is free, but in the future, I may add a fee (some proportion of trades) to use it.