I’m confused here. By your own writing the first depiction of that salute was from 1784 and you don’t provide any corroboration to the notion that it actually has any connection to any salute used by Romans. Jacques-Louis David could’ve painted the brothers dabbing or doing various stages of the Macarena (and if he did, he’d be closer in time to the popular use of those gestures than he was to ancient Rome)

How on earth could you possibly know that it was “based on” some maybe-kinda-similar-or-not Roman salute, but it also definitely wasn’t used in Rome? It originated there but wasn’t used there? That isn’t a coherent position.

It seems like you’ve established, with documentation, that an Italian guy saw and liked a painting made by a French guy that lived a thousand years closer in history to Gangnam Style than the fall of Rome. If you did some sort of rich analysis that extrapolates that into the history of the Roman empire it didn’t make its way into your article.