the fact that the tra la la lally elves don’t show up when frodo et al. arrive in rivendell implies a few options to me:
they are only there some of the time or at certain entrances (possible but prosaic, includes the potential for tra la la lallying as shift work)
elrond’s magic has some kind of built-in warning system and they cleared out to avoid potential danger (plausible and realistic considering that i don’t think team tra la la lally is either interested in fighting or skilled in emergency response)
the changing political climate and increased danger around rivendell resulted in all tra la la lallying activities being moved inside the valley (which begs the question of where, and if they just manifest barbershop quartet style beneath guests’ windows)
elrond didn’t know about it until bilbo unwittingly mentioned the experience over dinner, and all roadside tra la la lallying was brought to an end (either petering out without the allure of secrecy or simply by the power of elrond’s consternation)
the whole thing was contrived solely to troll the dwarves (elucidates the standout conformity of only these elves to historic ‘tree-dwelling hippies’ stereotypes)
bilbo fully made it up (this was revealed when merry and pippin cheerfully asked elrond when the tra la la lallying would start, and elrond said ‘gesundheit’)
assuming the tra la la lally elves are real, though, i’m so certain that it was a celebrían-manufactured conceit made permanent by her shameless use of a parting wish to a weeping and dock-bound elrond right before they were sundered for centuries. she was leaning over the rail of the boat to valinor shaking her fist and yelling that they had to sing the song she wrote and not some sad sappy poem meant to attract maglor. yes, with all the tra la la lallies. or else.
