[X] [ACTION] Actually start Mestina's training. Maybe she'll actually learn something useful once she gets kicked out of Grandmomma's lair, maybe you can get a measure of her personality. Or perhaps the interaction would be too grating, and you'd both leave in frustration. Who knows?
[X] [ACTION] Investigate whatever arcane mechanism or magic was responsible in keeping your lair afloat. Hopefully your blade would be more useful in this regard when it comes to information that they know and understand, but anything goes.
[X] [ACTION] Send a few goblins specialized in stealth to scout out the nearby settlements, and slowly build up information on darg distribution and supply. The sooner you can get this over with, the sooner you'll be rid of dargs in your territory.
[X] [ACTION] Find the remaining secessionists and eliminate them. The fact that this goblin army came so well-equipped meant that whoever supplied them has the capability to do so. Tracking them down yourself would ensure that this secession was stopped before any word of it could spread.
[X] [ACTION] Direct the goblins to start building fortifications against dragons. Not enough to actually hurt you of course – you weren't stupid – but bad enough that any dragon would be annoyed in trying to assault your lair.
[X] [ACTION] You've got a flying lair, you've got minions. Maybe now's the perfect time to rub your superiority over the other dragon that was currently lurking in Arnias. Or bug them. They're the only other dragon in the region, and Mestina is… well, her.