It seems like the old login servers don't even exist anymore (so I don't see how it'd actually verify unless it just checks a username's purchase status), but yeah that launcher does work for offline. (I still have my lastlogin file assuming it can't overwrite itself easily, but I don't think anything uses that other than the old launcher which can't seem to actually download the files because 404).

It's also interesting for the built-in modding, though it doesn't seem to be perfect. Also added an edit to my original comment mentioning parallel timeline mods. Though I'll just check out some classic(/revived) mods if I can get them to work.

@Stelus42