This stuff is still somewhat ambiguous, in particular Glossaryck's motivations and actions, so here's my take:>why did Toffee break Glossaryck's arm
because Glossaryck was goading him into doing so, I guess to set up Meteora's trigger to dipping down for the first time>why did he even send her there
In part to get Meteora's emotions high and desperate enough to induce a "dip down" moment, unlocking some level of magic, which seems really fucking early for an infant, since Star didn't seem to accomplish this until her teenage years. Also, I think, to take a dump on Toffee because of what he did to the book and the magic realm in present time. We can argue that Glossaryck made a time loop of sorts, but I think it's possible that this sort of time travel is inconsequential. Toffee hated magic and was already planning to take it over with or without Meteora's interference.>why is he even serving her
Either because he wishes to get a head start on the future queen/wand user, or because for whatever reason he recognizes her as the new owner of the spellbook. Which is currently one tiny shred of paper locked in Star's closet, so I'm not sure if Glossaryck's using other criteria or if he's still genuinely bound the book. This book was newly created by the 27th queen, and glossaryck was around before and after that, I think? Weird stuff.