Interesting topic,
LG. Though personally I think, as we Czechs say, that the dog is buried elsewhere, there may be something to what you are saying here. Elrond indeed seems to hint something, to me it rather looks like that he knows Frodo is going to take the task, but (hypocritically?) tries to push the thought backwards and tries to look, or maybe even convince himself to think, that anybody can be chosen yet. I just imagined the funny situation that often happens among schoolchildren or so: a request is raised and every high Elf Lord just sits and looks to the ground and thinks "Let's be silent and pretend that I am not here, Frodo will eventually take it".
Anyway, I believe with a little good will at least the second quote you provided can be relativised: Bilbo could write the sequel even if the quest did not concern Frodo personally, because the tale would still concern
Bilbo's Ring (which is actually why the old hobbit stood up in the first place, so the idea that Gandalf's response should be understood in this way is very plausible here).