re itwmd's replies that i can't quote without losing spoilers
It felt very much like grief to me, largely stuck and unexpressed until his wife's mum's death opened the wound. And also selfishness, or perhaps a self-absorption which can come from grief and not having enough spare emotional capacity to give others as much as we should. He's a widower who hasn't scattered his wife's ashes. Her mother, the person he can talk about her with as well as being his friend of many years, dies. Two sources of grief right there.
Grieving himself, perhaps. I understand the reality of this because of how much chronic illness has limited my life, it's very common for us. I'm not so sure about him. It may be that he became terrified of dying because of the trauma of losing his wife so young (he would have been around 47), or maybe he already had a fear of his own death and that was why he couldn't scatter her ashes. By the time of the film, these two aspects are intertwined and he finally faces both. Not that he's done grieving, it never goes away it just becomes more manageable.
It was selfish of him to ask her to move in with him and then change his mind, regardless of her reason. I don't think her reason for agreeing to was as limited as you do. I see nearly losing him as a simplifying and therefore deciding factor but not the only one. Living there would mean a big change to her life and he took her by surprise with it, so she needed time to think. But he put her through that decision and then tossed it away for his own preference.
The relationship felt typically Japanese to me, as they're presented in films and series. I tend to assume there are things going on we don't see, but maybe there aren't. There was a lot of cooking for and eating together, and he said something like the best thing is eating with someone you like. I've heard that elsewhere, probably Kinou Nani Tabeta? Anyway food does seem to be how they convey intimacy, at least in films.
No, I wasn't annoyed by the voice overs. Maybe because I have short sentence philosophical conversations with myself in the garden too? Maybe it's a matter of translation and we're hearing them differently as native and non-native English speakers? Over all, I'm not sure the film had as much depth of thought as it seemed it would. Or maybe I've just already given a lot of thought to death myself given the situation of my life. Anyway, those are my reactions to what you've raised.