Y'all too harsh on Tsukasa, she's in shock that that the first person she actually cares for doesn't remember such a significant moment of hers and its entirely likely that he had promised or agreed to something major in the missing time for example agreeing too become immortal too or something...
I'm pretty sure Tsukasa is the person who was meant to burn the Elixir on mount Fuji but she probably decided to use it herself and severely regrets it. Kaguya is the Kaguya who returned only to find no immortal Emperor. Nasa survived the Truck because he kissed Tsukasa and swallowed her...