make a new repl, but use the node 16 repl template by roblockhead.
literally search the language dropdown for 'node' and itll be there.
yes you can. download your code as a zip, do the thing, add pkgs.unzip to the nix config, upload your zip file, then run unzip on it (
unzip archive.zip in the shell).
forcing updates is a terrible idea. there's more than likely someone who's relying on some feature that got removed in v16, so just updating the version is bound to break something. that's why nix is better anyways, because you can just choose whether to update or not.