The halls of mandos and elvish "free will"

[Aiwendil]

I do not believe that this follows from 'omnitemporal.' Can't Eru be present in all time without being the immediate cause and mover of every event specifically?

Technically speaking, even a temporally linear God like that in Judeo-Christian theology would be 'present in all time'. That would merely mean that he/she/it exists from the beginning of the universe to the end of the universe. Elves are present through all time from their awakening to the end of the world. Humans are present in all time from birth to death.

The real question is whether a particular god is somehow beyond the unidirectionality of our time, superceding our strange macroscopic causality. As he is depicted in The Silmarillion, Eru is definitely not like this.