(a) is the real problem for many of the musicians who have vocally complained about this. If you look at most songs produced by record labels, you will see 5 songwriter credits, 10 producers, and a whole band to pay. Not to mention the army of recording engineers and the marketing staff.
Those people are doing real work, it's normal they're paid too. If musicians want more for themselves they could cut middlemen and produce and commercialize themselves their music.
Absolutely, and they are pretty much all equal participants in the creation of the sound you are hearing. It's just one worker (the headline artist) who gets all the attention.