[Aga-chan]

^Maybe I can help a little? ^^a

This part of "glitter"

Kimi no egao no wake
Kimi no sono kotoba no imi mo
Nee chanto wakatte iru tsumori da kara

kinda relates to this part of "until that Day...":

Nee kono egao no wake Nee kono kotoba no imi
Tsutawatteru wakenai Datte tsutaetai to mo
Omowanai kara Shikata no nai koto
Sou anata ijou ni wa daremo Shiranai
Hountou no Atashi wo

The later one I think is about people who think they know ayu - her fans, journalists, colleagues etc. Like we see ayu smile and think "Oh, she's happy~!". At the end of the chorus she says though that only "you" knows the real ayu. Like that "you" knows when her smile is for the public (photoshots, TV appearances) and when it's really out of happiness. Therefore "egao no wake" - "the reason behind a smile".
In "glitter" she says something similar the other way around. That ayu thinks that she knows/understands the real "you".