She wasn't stupid.
The Sundrop was clearly a key element in all of this, a lynchpin. Nothing could be solved without Rapunzel.
So she'd made herself out to be a new ally. She knew they didn't quite trust her yet, but she was at least gaining their respect and reliance.
And they needed her to guide them, if not too much. Rapunzel and her friends were skilled, but clearly not ready for what lay ahead, not yet. So she kept a careful watch on them, seeing them gradually assemble the pieces, and stepping in when they really needed it. Rapunzel's group needed to at least survive until they reached the true source of the Black Rocks.
After that, well...
She'd spent the past 25 years on the move, slowly but surely scraping up plans, building up skills and resources, all in order to fulfill the last orders of the now-disappeared King Edmund. (With everyone else gone, she was the only one who could.)
The Black Rocks had already destroyed one kingdom. Adira wasn't letting them destroy another, no matter what it took.
...Yes, Adira is my absolute favorite development to come out of Season 2, how did you know?
Adira is very mysterious, but as of now she's one of my favorite characters, so I decided to write this out. Of course this is just a theory I have now, who knows if the series will confirm it or disprove it.
Either way, I'm looking forward, immensely, to whatever the series plans to reveal about Adira.