As far as #2 i don't see this as much as you; there are a couple of different attachment styles but the actual chain rings are usually individually replaceable. Cassettes are not but (a) they are often machined out of a single piece of material or rivetted together and (b) the range is so big there's not really any point to swapping anything out. The entire drivetrain wears together so you can maybe replace the chain once before you should swap the entire cassette, chain and chainring(s).