Yes, something I should test and write up somewhere...
Indeed. Il all depends on the LB1836M motor driver. If you look at datasheet, you'll see that the driver is rated 400mA per channel. But the real limitation is the incorporated thermal shutdown. So you may go above this figure on one channel if the other is more or less idle, or for a short time (published curves stop at 550mA so this is a reasonable limit). Anyway, it is really sturdy, and I have yet to see one fail!