Celes does indeed know she is immortal and all that. but in her mind thats not a bad thing and she assumed Kaoru knew about it already( when in truth she don´t yet ).

(being young and having small breast is the best in her mind as they dont get in the way off her work and she dont like big once so she made sure Kaoru was like that forever to "help" her)

so when she said she would die of old age she thought she was joking.
as for Kaoru being a goddess, yeah that depends on your definition off one. she is"only" a human with godly powers, so when dose one become a goddess if you even can, To the common person she certainly looks and can acts like one.
really nice of you to include references to manga chapters btw.

most of the things i mentioned is from future chapters that i know off from the light novel and i'm about to lazy to go dig it up
Edit: derp forgot the spoiler tag xD