Is there a way to legally obtain a copy of Minecraft to join “offline” servers with?
Yes! If you already have a legal copy of the game (one that you paid for), Prism Launcher allows you to use an offline account (usually used for singleplayer when your internet is down), which lets you use any username. This is probably a feature in other launchers too.
I host my own Minecraft server and have
online-mode
set to false inserer.properties.
You can set it to true and use EasyAuth to allow offline players to join. This means online players get online UUIDs, and offline players have to log-in using commands and are given an offline UUID. You can also set up a global password, if you trust players won’t log-in as someone else.
There’s also EasyWhitelist for a name-based whitelist (by default I think whitelists are UUID-based, which probably don’t work with offline players).
Sorry for the late response. I don’t think the mods should be conflicting, just put them both in the mods folder and see if that works. IIRC Valkyrien Skies is required on both the client and the server.