[Grammar] Past perfect simple and past simple with 'before'.

Moncs said:

Write the correct form of the verbs in brackets.

I was embarrassed because she ______(arrive) before I _____(wrap up) her birthday present.
Book answer : arrived/had wrapped.
Students answer: had arrived/wrapped. Both are possible.

The book answer sounds correct but I'm really struggling to explain why.
If you take 'before' to mean 'a time earlier than', then the arrival is before the wrapping.

I also changed it to:

I was delighted because she arrived after I had wrapped up her birthday present. This is OK.

And then:

I was embarrassed because she arrived after I hadn't wrapped up her birthday present.

This sounds terrible but is grammatically correct right? No, it's not correct..
Any help much appreciated. ;-)

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