Hmm... if the car draws 10A and the headlights draw 15A, a 50A DC/DC would suggest you have 35A (just under 500W) to play with.
I find it unlikely you could damage the DC/DC with a 12V load. I've never heard of a DC/DC converter failure. That thing has a MASSIVE heatsink and fan on it, and it's not going to overheat. I'm sure it's got over-load protection as well, which in the worst case would drop you down to battery voltage.
I've used my DC/DC several times to charge my 35Ah 12V battery from flat to full in one sitting, a procedure that can kill some alternators.