The spine creature has nothing to do with Nidhogg as it is not a black dragon gnawing at the roots of a tree.
Jormungandr's appearance doesn't make the beginning of Ragnarok. The beginning of Ragnarok starts with Baldr's death, which is why Frigg made all of creation promise not to harm him (except the mistletoe). Loki tricked Hodr into shooting Baldr with a mistletoe arrow, then disguised himself as a jotunn who refused to weep to prevent Hel from releasing Baldr. The giant spiney Titan Eren transformed into looks nothing like Jormungandr.
Reiner being Thor and defeating Eren is speculation.
Ymir is MALE in Norse mythology and he's the progenitor of the jotnar, not a goddess or god.