Bill dies at the end
The immortality sequence was simply a way to say that his amnesia creates a mental state where he "lives forever" because time doesn't seem to pass. Bill stays mentally the same "forever", until he finally dies.- No, it's not that simple. And fortunately not that depressing. In this final moment he's having a complete and sudden realization how beautiful and amazing his world is and gives himself what he wanted since he was a child. To explore his world. The world he fell in love with so long ago. He subconsciously understands that the horrible things he thought happened in the past didn't really happen and that they don't matter anyway. He makes peace with the past. And by saying that he lived and lived until the last light went out, the narrator is saying that he lived until his last dying breath and made peace with his world and death. So in a way the ending becomes true in it's own way.
Bill's ex-girlfriend's name is Naomi
Bill attempts to say a name, likely Naomi, during It's Such a Beautiful Day. This is when he was knocked out during surgery and the medics are asking him questions.- "Do you remember her?" "Naaaam. Naaaomm. Mmm."
Bill was still in love with Naomi but lets her go.
"Do you remember her?" It's the only thing he does remember. And then he sees a flashback of her laying next to him. With his memory loss, he's inadvertently accepting the fact that she's with someone new and lets her go to let her live her own life. The entire animated part is in Bill's mind.
In one of the scenes, the narrator talks about how Bill might imagine things in his head. At the end, everything seems different to him, more detailed,represented by it being live action. At the end of his lifespan, does Bill start to realize the true things going on?