Can a pro se litigant be represented at a trial by another non-attorney pro se advocate?

No. I'm only writing the remainder of this response to meet the minimum word count.

How often does a homeowner get full recision on a new home full of defects in arbitration?

Unlikely. The failure to find issues during the inspection process will hurt you.

Do that seems like a law suit anyone can take?

Google the drug name and class action to see what lawyers are involved. Good luck.

What steps can we take to have him declared dead through the court?

Hire counsel to file a petition to have him declared deceased. I struggle to understand why this hasn't been done already.

Is there anything I can do to prevent something tragic from happening? I'm just a grandma who doesn't have 5,000 to get help!!!

Sometimes you need to stay out of the way and allow fate to work. If the child lacks the necessary daily needs then hire counsel.

If you violated the law hire counsel and see if they can help. The fact that your gps was part of the story is a non factor.

Can I sue him for that money or do I have to let it go entirely?

Move on. That isn't enough money to hire counsel. Don't help him or her going forward.

How to handle a hit and run? It could deal with injury

File a claim on your insurance and move on. If you don't know the other driver you have no case.

Car accident and personal injury

Sounds like you have liability only. If so you gambled and lost. Expand your insurance going forward.

Any accountant or tax preparer should be able to help.