I would speculate that GAW likely overpaid for some number of blocks for some reason (maybe orphaned blocks) and the negative amount is due to this, or maybe they previously allowed you to withdraw more then they should have.
Nope. You're getting everything wrong. First of all, hashlets are not actual miners nor they represent actual hardware. You even agree to those terms by loging in to zencloud. Given that, we know that the hashlet scheme can't be facing the same problems a regular mining pool faces. If they come up claiming that they faced a problem like the one you described, we can know for sure that it was just a cheap excuse.