I-485 Aplican, Preparer or Interpreter for 8 years old son application ?

You may indicate preparer for your sons forms.

Applying for citizenship out of state

No this is not advisable and should not be done simply for the process of faster processing. Be patient with this and in general.

Yes this may be true but there is nothing you can do about what service center is processing your case.

How can I avoid being stopped for additional interview at the U.S. Border Protection every time I travel to the US?

If you have a criminal record, despite expungement, it will be disclosed and reviewed by port officers, possibly leading to secondary inspection.

Yes this will impact future immigration petitions.

I'm in a limbo, what are my options to stay in the US?

Unfortunately you are not eligible for a work permit while the U visa pending. Consult with an attorney for guidance re: your case.

Do I need to fill out the I-485 Supplement A when my eligibility category in the form I-765 is (c)(9)?

No this supplement is not required in your particular case.

My husband green card inteview was in Trinidad on the 2/06/2019

If the review is completed you may contact the embassy directly to inquire about decision.

Is it safe for me to go back home having an EAD/AP combo card?

You may travel on the combo card while your case is pending assuming you have no grounds of inadmissibility not disclosed here.

Yes you should enter the information from your initial entry.