How do I drink as a Earthen to get Mana Back?

All of it is required.

They’re usable now – you can eat them now, in other words, they must have hotfixed it in overnight.

There are some issues with it, still, because the three gems only cover 3/5 possible colors, which doesn’t cover all of the potential buffs (which means that drinking to restore mana using a Belga gem of a color that doesn’t match the preferred buff will overwrite it, apparently).

Some of the details are in this thread, towards the end: What are your thoughts on Earthen racials? - #142 by Pandamns-bleeding-hollow

Still, it’s definitely a big step in the right direction.