1 the max load for any circuit is ultimately decided by the wire guage. Put too much on it, and smoke appears.

2 you can use both, but why would you? If you put them in parallel, then you'll double the power load. With 30 A fuses, that would likely fry wires ( in case of a short ). In series, and then it's a contest to see which one goes first. Personally, I'd choose the breaker.

3 never use locktite on an electrical connection. It can act as an insulator, much better to use an electrical washer: they have little fins all around, like a star. They promote continuity, and function as a lock washer as well.