What does USCIS mean by tax records when you have not filed taxes?

You may submit a statement to reflect that you were not required to file taxes.

Will they hire me without papers?

Employers generally require proof of status and work authorization in the US for employment purposes.

Yes the I797 serves as an extension of your GC status.

Interview Cancelled?

This appears to be an error in the system as you were already issued your GC.

Contact USCIS directly to reflect appropriate receipt information.

Can my aunt go back to her home country after completing biometrics without waiting for the results?

Yes she may leave at any time and her case will be considered abandoned.

More information is needed to make such a determination. It is best to consult with an experienced attorney for guidance.

You may request anything from your attorney and should do so privately. That is between you and your retained counsel.

Yes you may file the RFE late, although not initially advisable. Best to work with an attorney for such late filing.

He may indicate N/A for this section as it does not apply to beneficiaries outside of the US.