So, Hydaelyn called Minfilia back into Y'shtola's Flow for the sole purpose of pulling her to the Aetherial Sea and having her sacrifice herself? This seems... shady. Anyone who still thinks Hydaelyn's not treating us like tools (or chess pieces) had best wake up. Minfilia was in no danger - what was the purpose of having her get swept up in Flow and keeping her in the Aetherial Sea? Getting a mouthpiece? Hydaelyn never had trouble speaking to us before... and once our Blessing was restored, has not had trouble since. Something's not right...
Her story, while all we have to go on, is also very lacking detail. "Zodiark got power hungry, so I banished him" is... way, way too simplistic to be the whole truth. "Keep fighting and foiling the Ascians"... our mission is not changed. Yet, I cannot ignore our Echo flashback into the Warriors of Darkness' memories, with them being told that killing the Ascians will not change fate.
Damn it... as always, we need more information. But... if Minfilia really is gone, then... I take great issue with that, doubly so given the shady circumstances of her "sacrifice," and while I don't want to doubt the Mothercrystal or turn my blade on her...
Even more apparent that he's a specter / shade / ghost / etc. now than ever. Any normal dragon cannot switch between a human and dragon form - they're just dragons. Estinien-Nidhogg, however, can switch between forms at will, and the Quest Log continues to refer to him primarily as a shade.
Pry the eyes off him... or just destroy them. We can save Estinien... we have to.