So he would have got a future with Cossette since he wanted to go with her where "eternity can't reach them"?
Which contradicts to the whole main point of the story of him sacrificing himself for her good? Which didn't matter in the end because he died anyways and smiled for some reason?
The only purpose of his death would have been to free her, which he failed to do so... So why did he die with a big grin on his face?
In my opinion the Manga ending left faar to many point unanswered and is thus not only a disappointment but also a inferior to the ending of the Anime adaptation, which wrapped it up in a nice way.
I really liked the story buuut... I take it that Eiri panicked at the end & couldn't sacrifice his life, thus the ritual failed & Cossette remains "imprisoned by eternity"?