So the fate of an entire empire is a test for one pony? That's pretty reckless. And if you fail... well gee, you fucked up an empire. But hey, no pressure.
I actually get the impression that Twilight is rapidly reaching a point where, if she snapped, she'd be a bigger threat to Equestria than Sombra would've been.
Her talent with magic has been growing by leaps and bounds, to the point that she's achieving things that would previously have required Celestia and Luna—characters of mythic levels of power—to match, and yet seems oblivious to this fact. There's a couple indicators of this in the episode, too: when Celestia and Luna are shown standing together, they have dramatic Sun and Moon backgrounds behind them, and when the camera changes to Twilight, it shows a starburst background in the same style.
What with the exchange between Celestia and Luna and the fact that they said this had to be a test for Twilight specifically, I suspect the entire thing was less about 'we need to save the Crystal Empire' (which they probably had a contingency plan for) and more about 'oh, hey, look, a crisis situation which provides a convenient test case to make certain this increasingly powerful individual is capable of handling pressure on her own without snapping and trying to destroy the world.'
I mean, Twilight has not historically handled stress well. In fact, I'd go so far as to say that her mechanisms for coping with stress are
hilariously insufficient to her power level. She brainwashed half of Ponyville into fighting over a doll because she was worried about not having a topic for her weekly letter. Heck, just in this episode, she blew up the library in the first five minutes
because Spike suggested she might be overreacting. And let's not get into the whole time travel mess.
So, there's my take. I guess we'll see what's up.