Midgardsormr is the DRG LB3 since Heavensward. His eyes are in his body in Silvertear Lake. They're just mostly divested of aether cause his original body is dead ever since it coiled around that Imperial Dreadnaught and blew up in 1.0.

They are still intact and slowly but surely drawing aether in, regenerating him. He even wakes up to aid the 8UC in that one short story.

Shinryu is just a primal based off of his original appearance, based off how he looks in O10 (O11 is Omega).

We have a lot of Nidhogg in our DRG skillset because we got setup to be an Azure Dragoon in the ARR DRG quests. The Azure Dragoon draws their incredible power from Nidhogg's Eye. We then gain Hraesvelgr's eye once when we reclaim it from Nidhogg in The Aery. Then again when it's bestowed upon us to give us equal footing to him, to beat him on the Final Steps of Faith (where he fought us both as himself and as corrupted Estinien).

Shinryu's original incarnation born of the Baelsar's Wall sacrifices was also incumbent on having Nidhogg's eyes in the mix.

Ever since Final Steps of Faith, part of Nidhogg persists within Estinien due to the fusion of their souls into one vessel, and this is is made apparent several times along the story, usually under humorous guise. (Estinien's intense love of certain meats, his reference to Alphinaud and WoL throwing his eyes off the bridge, naming his lance "Nidhogg." His displays of power that harken back to Final Steps of Faith powers)

He's definitely the "Nidhogg DRG" but the WoL still has entanglement there, as well as the auspices of Midgardsormr, who all of the other great wyrms are derivative of.