> How to handle Interrupts in C? Will they perform as callbacks that await some return values or do I have to use inline assembly to process them correctly;

Not to disappoint you, but you should try doing some more low level programming or dabbling with some existing OS code to have an idea how this kind of programs look like. Maybe having a look at Minix for a reference of simple OS?

Have you thought about targeting ARM? Its architecture may be way less trickier than most Intel CPUs.

Well, good luck with that. Worst case scenario, you'll end up reading lots of interesting resources.