Oratory consists of a lot of tricks that not necessary for one-on-one: eye contact with more than one person at once, projection, inflection, dictation, body language over facial expression, overstating facial expression. Some of it, like inflection and diction, are useful for one-on-one and that’s a FoRK thing.
“Likewise you should throw dice at a player who suggests that his charming swashbuckler-man is just so fancy-pants that he should be allowed to use Oratory to talk to one person because he’s playing to the imaginary audience he carries with him.” Yes, though FoRKing Oratory and Conspicuous fit the scene well as swashbuckler uses the associated knowledge to keep the one person’s attention on him to the exclusion of all else, including flaws in his argument.
Rhetoric, in my mind, is the ultimate FoRK for social characters as it slots into Persuasion, Oratory, etc so well: it is the bullet while those skills are the gun. One could use it as a primary social skill but it is basically the Spock option: a clear outline of logic but no feeling behind it. Unless the person/people are completely rational in decision making (cynics say never) then a +Ob is warranted.
“What skills works on the GM, it seems only Soothing Platitudes and Falsehood functions to any degree”
Not sure what you mean here… like roll dice and the GM is actually affected? If that is the case I find a sock full of nickels works wonders on getting him to change his mind. Oh, that’s technically Intimidate isn’t it?