“Snickering”, as far as I can tell is just the American version of “sniggering”. I can’t track down the etymology of the split and so I can’t see any huge mentions of “it sounded a bit racist so we changed it” but I guess it seems plausible given things like “titbit” being changed to “tidbit”.

While someone saying “niggardly” is a bit suspicious given that it’s a pretty archaic word that most people haven’t even heard of, “sniggering” is a pretty normal word to use in Britain. I imagine if some racist wants to start using it as a bit of coded bigotry they could, American-branded homophobia has already ruined the word “fags” to describe your cigs!