Optional flat adverbs with more than one meaning

In case of the following adverbs, there is often not much agreement among native speakers whether the flat versions may be used at all, or in what context or "level of style" - even if they are real flat adverbs and their usage is grammatically correct. To make things even more complicated, in some contexts or idioms or for a distinct second meaning, either the regular or flat form are mandatory.

clear, easy, fair, sharp

A: The regular form is required for a distinct second meaning

clear [optional/split] - Both forms are possible, but more often, "clearly" is the safe choice. Then again, e.g. in Adele's song "Rolling in the deep", the line "I can see you crystal clear" would sound extremely bulky if Adele had chosen for "crystal clearly". One could argue, though, that "crystal clear" does not describe the vision but the picture, then it would indeed be an adjective and the regular adverb would be simply wrong.
The following example could be explained both by saying that it is a short, informal sentence not burdended with the regular form, or that in fact the quality of the perceived sound ("loud and clear") is described:
Q: Do you hear me?
A: Loud and clear.
There is also a second meaning of the adverb "clearly": In sentences like "You have clearly crossed a line" it is used figurately and means "doubtlessly".
mentioned in "When adverbs fall flat"

easy [optional/split] - There are certain idiomatic phrases such as "Take it easy!" and to go easy on someone in which "easy" is used as a flat adverb. There is the song Words don't come easy by F.R. David, and there is the term easy-going.
Apart from that, however, it appears that the regular adverb is used more often than the flat version:
E.g.: Information on the subject is easily obtainable I can easily find what I'm looking for (easily as in "without any difficulty").
It is important to note that there is second meaning of "easily" as in "doubtlessly": Venice is easily the most beautiful city in Europe.
mentioned in "When adverbs fall flat"; examples taken from Cambridge Dictionary

fair [optional/ split] - Both the flat form and the regular form can be used to express the meaning "right, honest": "To treat somebody fair" is possible, especially in an informal context, and so is "to treat somebody fairly" (the preferred version for a more formal context and probably for British English). While both the flat and the regular form are possible, the flat form is considered "informal" and clearly less accepted. On LEO, the regular form is supported by British and American native speakers (#3+4), while the flat form is rejected by a Briton (#55) and accepted as "OK, but only informally" by an American (#56).
The regular form is required for a second meaning as in "somewhat", e.g. in "it was fairly convincing" or "they treated him fairly well".
mentioned in "When adverbs fall flat"

B: The flat form is required for a distinct second meaning

sharp [optional/split] - "sharp" and its normal form are interchangeable ("Dress sharp", or "Dress sharply"), but the flat form is less widely accepted. On LEO, the flat form is rejected by a Briton (#55) and accepted by an American Cambridge Dictionary gives a whole variety of meanings for "sharply", such as suddenly, able to cut, angrily, clearly, fashionable, and clever, indicating that this word is indeed widely used and that in various cases (depending on the context and particular meaning) the regular adverb might not merely be the safe, but the only correct choice.
The flat form has got a distinct second meaning in phrasings like "Show up at eight o'clock sharp."
taken from "Flat adverbs are flat-out useful"; also mentioned in "When adverbs fall flat"