A petitioner may attend I-485 adjustment of status interview for parent of US citizen and also be the interpreter. Clearly there will be no issue...

It depends on what the G56 documents.

Application of old 321 may be tricky and facts heavy. It is best to consult an experienced immigration attorney, rather than read the regulation.

For I485 application, RFE for derivative does not impact the principal. i

Yes you may financially sponsor your au pair for studies in the US.

F1 is valid for DOS. The same for derivative F2. Students on valid F1 do not accrue ULP on return to US. However, speak to your university DSO...

With a valid visa, yes. EADs are employment documents, not travel documents.

You may submit FOIA request to multiple agencies depending on the information you need.

Your matricula and Mexican passport are sufficient documents for travel inside the US.

Your AP may be used for reentry while AOS is pending. The I485 approval while traveling does not invalidate the I131.