Please check my por/para homework

[AngelicaDeAlquezar]

#5: You're talking about a woman who is not very young and still looks good.
If you use "por", you make her age a cause for looking good, which makes no sense, since older women are supposed to look bad.
Check definition No. 5 of "para" at the DRAE.

#16: No, it's not idiomatic. It's a future action.
Check definition No. 24 of "por" at the DRAE.

#17: I'd use "por" then, which would mean that you're not sure about the exact date.

But as Hernán says, for passing exams, write what your teachers say is correct.