a. grounded negative chassis is a convention, it can be a grounded positive instead, but every connection has to be reversed. ground is a concept, a point of reference, but not a physics reality.

b. the current passes through the battery positive terminal and the fuse, and so it doesn't matter what the return path is; a negative cable or chassis. the fuse will still blow on fault. the fuse has to be part of the circuit for it to blow.

c. there is no alternative route, the current must pass through the fuse. it may also pass through other things, but this is not important; the fuse will still operate if the current and time allow it to.

d. the alternator has a cable connected to the battery positive, and either a cable connected to battery negative or the chassis is used as the path. it works because the alternator generates a voltage higher than the battery, and the voltage carries power into the battery. during charge, the battery is a load with respect to the alternator.