I don't know of another drain. If there isn't another plug that's just not covered in the manual the only thing I know of would be pulling the water pump. You should probably inspect the water pump anyway.
Can you get a picture of the broken off plug?
I don't know if draining the radiator would provide enough suction to open the cap and allow it to siphon the coolant from the reservoir; I've never left the radiator cap on while draining the system.
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