There is a comprehensive article looking at various aspects (including historical preferences and shifts) of adverbs-that-resemble-adjectives here; it cites other good sources. The question of which of these so-called flat adverbs are licensed by some style guides is also partially addressed. I'd just add here that sometimes, flat adverbs have a different sense from their related -ly forms:
We went to Rome, and then flew directly on to Rio. (ie as quickly as possible).
We flew direct to Rio. (ie without landing mid-journey).
The use of the -ly-less form is best usually regarded as informal though, in my opinion.
Addressing your second question, I'd reiterate that I think it's about time degree modifiers:
He drove a real(/ly) fast car.
He's plumb loco.
He drove real(/ly) fast.
and other 'secondary modifiers':
It was chillingly realistic.
Time passed excruciatingly slowly.
were recognised as having very different functions from words modifying verbs.