ShB story did not go the way I thought it would. (Spoilers)

[Yshnal]

Hydaelyn was never intended to be evil. Primals are not evil per se (see Shiva, Ramuh, or that certain one in the kojin tribe quests), and both Zodiark and Hydaelyn are special cases at that, too, in the sense that -unlike other primals- they don't actually seem to be negative (drain) to the flow of aether just by themselves. Or at least Hydaelyn doesn't do it, as we don't know what would Zodiark do. Would he keep trying to get more powerful and, in the absence of sacrifices, drain aether like most of the other primals?

On the other hand, the Echo is not given by Her to the WoL. What Hydaelyn offers is the Blessing of Light, which is like a super power that she gives us, AKA the power of the crystal. The Echo is independent of it, and it's a remnant of power from our unsundered selves (Echo users are parts of the split souls of the Ancients) that protects us against tempering, allows us to see other people's past/aether, and other unknown perks, if we believe the Ascians. Also, it will be interesting to see if it has gotten more powerful now that we have absorbed another piece of our original soul, meaning Arbert.

Now, about what was happening with the First. We know from very long ago that Rejoinings are not for Hydaelyn to get more power, but quite the opposite. The Ascians were the ones behind the massive light surge in that shard, as it was what working well for their objectives. In other words, if we hadn't done anything, a Rejoining would've happened, followed by a new Calamity in the Source (our world) after it would've absorved the First. The objective is to get the world to what it was pre-Hydaelyn, bringing back Zodiark and getting rid of her. Emet and the Scions explained it way better than me, so you could revisit the cutscenes at the inn if you don't fully understand what I said ^^'

Moreover, the story kinda implies that we aren't actually a pawn of Her, but that we (the former 14th member of the Council) might have been the ones that originally summoned her to defeat Zodiark and sunder the world. Probably to avoid another End of Time happening due to the raw power that the Ancients had, unless there's more that we don't know yet (which might be where the MSQ will go once we're done with the Ascians, but we'll see). I think that that was an actually cool turn of the story, and a surprise for most people.