At the Samui immagration, you have to first get the 30day extention and if the situation still allows for 60day covid extension. You can do so 7 days before the 30days is up.
you will receive a certificate plus your covid tests results on paper from the hotel at check out. I was never asked to show them when checking in with Bangkok airways.
no you should be able to apply for the STV through your nearest Royal Thai embassy or consulate when it's made available. No need to use agency or company.
You don't have to anymore. It was required earlier when they started letting people apply for visa's again but no need to upload anymore. Fill it out just in case any officials asks for it. But when I arrived 12 days ago no one asked to see it.