Translation:Corinna is not a girl, but is a woman.
The English suggested translation is awkward. English do not really have elliptical subjects.
Yeah, it really sounds like a terrible word-for-word translation.
How would you say "Corinna is not a girl, but the woman is"? (I know it makes no sense)
My guess is, that in order to say just that you need to be more specific (although you are right, it does not make sense): Corinna non est puella, sed femina est puella.
They could probably have solved that phonemic problem just by going with Ecclesiastic. This attempt at Classical makes it sound like a constructed language, especially as the speakers are native to a language that has no phonemic connection to Latin.
When should you put "est" at the end of a statement and when is it acceptable to put it within the statement?
If you have a negating word, like nōn, then it follows the negating word. Otherwise, it doesn't matter. You can order sentences however you like to stylistically.
"Corina is not a girl, but is a woman" is given as a correct but I don't think that's grammatically correct English.