The more knowledge you have about bike repair before you leave, the better. The main things I would concentrate on would be learning how to replace spokes and true wheels and adjusting the brakes and derailleurs. A chain tool will rescue you if you brake your chain - all you have to do is shorten the chain. That usually means you have fewer available gears, but you can keep rolling. The suggestion to carry extra bearings doesn't make sense since your bike has sealed bearings in the headset and bottom bracket. Disc brake pads don't take up much space, but if you install a new set before you cross into Mexico you most likely won't need to replace them again anyway. I wouldn't bother with a spare rotor - in an emergency you could always try to bend the rotor back and hopefully you wouldn't damage both brakes simultaneously.

In the end it doesn't really matter. Half the fun of going on big adventures is what happens when things go wrong. Just get out there and ride!