We know that they can do things on their own. So it's not certain that they would do absolutely nothing when left alone. As far as we know, there have been no Rosharan scientific studies done on this front, so we can say nothing for certain. For instance, perhaps after a long enough period without orders, they revert to basic animal instincts and start foraging for food.
This is just a non sequitur. You asked for better ways to treat slaveform singers, not how to treat human slaves.
As I said, if this arrangement is selected, the cost of food and board would be deducted from their pay. Do you have any evidence that only paying for basic necessities will cause the economy to collapse?
Besides, another option that I pointed out was to put them into self-sustaining farming communities. You don't have to pay or feed them, and depending on how tyrannical you want to be, you can set up population controls so that you won't have to expand the farms. And if there is any surplus, you can take it and give them money for it.