If you want to stay whitehat and avoid jail, committing crimes is a bad idea. For criminals, blackmailing is part of their M.O.
There's an international market of brokers for zero days, but this specific vulnerability is less in demand.
I doubt that the information found at Facebook legal team could be used by nation-states (but perhaps I am not thinking creatively enough). I can imagine it being used as leverage by a nefarious nation-state, or informational by anti-trust dept. but it would be thrown out of court (therefore only viable for parallel construction). In the case of leverage the nefarious nation-state would feel the wrath of Facebook and/or US government. A country where Facebook has near zero adoption and already on bad terms with USA while within power vacuum, perhaps. Russia has Vkontakte, North Korea and China don't use Facebook either.
Regarding PGP, you don't have to use your real name. You can use an alias. You can sign each other's keys at a crypto party.