Maxperson
Morkus from Orkus
I'm not saying it would be common knowledge for the masses, but in the specialized world of spellcasters, such information would be knowable arcana. It might take a roll, but it wouldn't be something that could be kept a secret, especially from warlocks who would look into such things.I don't believe "every dirt farmer and street urchin has memorized the PHB and MM" has ever been official, so there is no reason to believe that anyone knows any "cosmic secrets." Also, patrons pick warlocks for charisma, not wisdom or intelligence, so it is unlikely that the warlock can detect a lie of omission or could identify the truth out of scores of wizardly tomes on arcane deals that probably split hairs on all manner of things.
In my mind it's more independent than that. John Doe contacts an archfey and makes a deal to do X, Y and Z in exchange for some power. Nancy Drew contacts Belial and makes a deal with him for abilities that let her be a better detective in exchange for A, B, and C. Jane Doe, annoyed at her husband John's constant pranks, contacts Cthulhu and promises to bring about the end of the world in exchange for power to get back at John.Your second question is more interesting. I think of the pain as basically being money (thus the bank reference), although it is possible that it is a tasty snack for the GOO's, art for the fey, and a source of power for fiends.
That leads to a question, which I have not really thought of until now, did the warlock thing arise independently among these groups, or did one group invent it and the others appropriate it? Given the large amount of uniformity of it, appropriation does seem likely. If some GOO invented it as a way to get pain M&M's, an early warlock might have fallen into the clutches of a fiend or archfey who decided that he/she/it could get something he/she/it wanted out of the deal and replicated the warlock template.
D&D just needs to make the warlock base class uniform, so all three get the same basic abilities.

