He doesn’t remember the night of the first comet at all. I could be wrong but I really don’t think would have just moved on without addressing the year of memories. They didn’t talk about or show their couple (engagement?) rings at all or the original confession. They only talked about the night before, the missing three hundred days, and the future. Rihito never pushes Arata to remember anything, so he wouldn’t bring it up if Arata can’t remember. I have read some amnesia trope stories and they freak out and gush about how sorry they are for forgetting and immediately start talking about their memories. They never do that. He only remembers the night before because “give it back” refers not to Arata’s memories but his ability to move forward and make new ones. Neither of them wished to forget anything, only to keep reliving the same moment.
Lol not the expertise. Yeah they are because Arata accepted Rihito’s confession the day before the comet came back and Rihito asked to undo his wish (Rihito confessed everyday and Arata always accepts). Overall I think it’s a happy ending with or without memories since they are together and they even hinted that Arata might be able to regain some memories in the future.

I’m kind of sad that he still hasn’t recovered the memories of the year they were dating nor the three hundred days. I understand why he wouldn’t remember the three hundred days of amnesia but not remembering the year before is crazy since even the grandma got her memories of her daughter back. I hope they come back over time.