What does it mean?

I would suggest consulting with an attorney individually to assist you with this matter

Can I file a case?

Yes there are whistleblower laws that are in place.

Should I still show up at court?

Why not check with the court itself? Or online if you know the court number? Or call you opponent for the details of the place, date, time and...

How is a persons legal status not relevant in a child custody case?

Read the constitution. It says it right in there.

What can I do about this I have a lawyer that I get a hold of.

Go over to the attorney office for a meeting.

The US government will recognize and address where mail is delivered.

You may sue in small claims court if you think you are not going to ever get paid

Is this true?

Find another person to rent from, and hire a lawyer too

I think the doctrine you are attempting to apply is collateral estoppel.

Should I go to court for a modification?

Yes, but only if there are changes that support the request.