I did apply TRV for my father notva supervisa, I would of mention supervisa if I did. Commisioning a letter is just a very small step with very minimal fee that can help his application. He has nothing to loose bu doing so.
You write the letter then bring it to the city hall and asked for the commissioner of oaths to commissioned it. Make sure your letter contains information about your household and accommodation support.
Yes, I think for parents and grandparents, the proof if funds are from the host while for other situations, economic ties and funds are to be presented by the applicant
Actually, the worker has to prove that he is willing to return to his home country after his contract hence proof of ties to home country is needed like family ties, employment prospect, financial stability