Did you even fucking read the manga?
- There was a CHANCE that she wouldn't be able to sing again IF the cancer had spread to her vocal chords. Mitsuki says that even if there's the slightest chance that she'll lose her ability to sing, she'd rather die than risk it.
- The cancer doesn't get worse while she's transformed because it doesn't exist during those time periods. It did not get better, it's simply "put on hold". Since she's Full Moon probably 50% of the time, the cancer progressed half as quickly as it would have had she not been transformed, so it wasn't quite as bad as "oh my god she's had cancer for a year".
- When the fuck did they ever say being Full Moon made her cancer worse? Ever? Don't make shit up faggot.
PS: Anime sucked compared to the manga. The manga actually tied off all its loose ends pretty well with minimal LOL DEUS EX MACHINA shit. Everything that seems too perfect was actually hinted at all along if you read it a second time. Like Takuto being in a coma...you see Dr. Wakaouji calling someone after he sees Takuto the first time, and he says "you're right...he couldn't possibly be here." He's calling the hospital to make sure Takuto's comatose body is still there.
lol i can has reeding comprehenshun nao?