What documents are required to cancel an ED visa and apply for a COVID-19 extension in Thailand?

Hello,

Few questions for me today :

1. Which documents do I need for cancel my ED-visa? I know about the letter from the school (Duke Language School) but is there something else? I plan to set the cancellation date to next Tuesday 26th October.

1. Which documents do I need for ask the covid-19 extension of stay? I plan to go ask for it as soon as I made the cancellation of my ED-visa (tomorrow or Monday depending how long does it take to Chaengwattana to process the cancellation).

1. Can I ask for a Non-B visa from there : a cancelled ED-visa obtained in-country in march this year (date of cancellation set to Tuesday 26th October) and a current covid-19 Extension (start after the day of cancellation of the ED-visa I suppose)? The company I gonna work for is based in Bangkok but she outsource all the administrative stuff to a company named "ATA service". That company told me today they can't get me a Non-B visa with my actuel visa situation. So, I'm a bit confused since it's since that some others people (with a similar situation) already successfully did that process, and I think they just look at it like "no change from ED-visa to Non-B visa", without thinking about the cancellation + covid-19 possibility. But at the same time they since quite competent for their stuff... I don't expect a clear yes/no answer, but some feedback from people who already did it would be appreciated. Also, Do you think it's worth it to try it anyway (asking the Non-B visa) no matter what (even if the company ATA service believe it's not possible at the moment)?

Thank you.