Question 1; I'm not sure what your setup is, Stock GM, ford, etc. But the key switch normally powers the solenoid of the starter. I rewired my car with a Ron Francis harness and am using a GM steering column, ignition switch and high torque starter. I believe the solenoid on mine has a 30 amp fuse on it. Yours may be different.

Question 2; I am going to assume you would like to use a breaker so you don't have to replace a fuse if it blows. You could use both but in the event of excessively high current draw whichever device (fuse or breaker) is "weaker" will trigger. Even with both being rated at the same amperage there is a +/- tolerance. With a 5% tolerance @ 30 amps this could be as much as a 3 amp (1.5a for each) swing. A fuse over time will be affected by vibrations common in a car, especially the old glass tube type weakening them.

Question 3; As others have said, as long as your not using the bolt or nut as a ground or other electrical connecting point, I don't see why you couldn't use Locktite.