Just because he's trading using your API keys, doesn't mean he can't drain your balance dry.
By shuffling money through low liquidity trading pairs there's a chance he could empty your account and get away with your money.
This should only be possible for accounts with a huge balance, other than that it probably wont happen.
Yeah it's probably worse with leverage. One bad trade with overconfidence or misset stop losses (even something spelt wrong) you're just asking for trouble.
And 50% fees is high since from your end there's no risk... You might want to make a telegram channel and get people to use something like cornix.