What are the requirements and processes for obtaining a Non-O Visa for Guardianship of a Thai Minor?

𝟭) 𝗜𝘀 𝘁𝗵𝗲𝗿𝗲 𝗮𝗻𝘆𝘄𝗮𝘆 𝘁𝗼 𝗴𝗲𝘁 𝗮 𝟭 𝘆𝗲𝗮𝗿 𝗠𝘂𝗹𝘁𝗶𝗽𝗹𝗲 𝗘𝗻𝘁𝗿𝘆 𝗿𝗶𝗴𝗵𝘁 𝗮𝘄𝗮𝘆 𝗼𝗿 𝗺𝘂𝘀𝘁 𝗶𝘁 𝗯𝗲 𝗮 𝟵𝟬 𝗱𝗮𝘆 𝘀𝗶𝗻𝗴𝗹𝗲 𝗲𝗻𝘁𝗿𝘆 𝟭𝘀𝘁 𝗮𝗻𝗱 𝘁𝗵𝗲𝗻 𝗹𝗮𝘁𝗲𝗿 𝘂𝗽𝗴𝗿𝗮𝗱𝗲𝗱 𝘁𝗼 𝗮 𝟭 𝘆𝗲𝗮𝗿 𝗠𝘂𝗹𝘁𝗶𝗽𝗹𝗲?

No there's no way to get a one year multiple entry VISA once you get the 90 day single entry

You would come here get stamped in for 90 days, have the legal parental rights decree enter granted by the family court here OR the local city hall (if the child is old enough) bank 400K baht in a thai bank account and apply for a year "extension of stay" at the imimgration office.

𝟮) 𝗜𝗳 𝗜 𝗱𝗼 𝗮𝗻 𝗘 𝗩𝗶𝘀𝗮 𝗮𝗽𝗽𝗹𝗶𝗰𝗮𝘁𝗶𝗼𝗻 𝗳𝗼𝗿 𝘁𝗵𝗶𝘀 𝗱𝗼𝗲𝘀 𝘁𝗵𝗲 𝘃𝗶𝘀𝗮 𝘀𝘁𝗮𝗿𝘁 𝗳𝗿𝗼𝗺 𝘁𝗵𝗲 𝗱𝗮𝘆 𝗶𝘁'𝘀 𝗴𝗿𝗮𝗻𝘁𝗲𝗱 𝘁𝗼 𝘆𝗼𝘂 𝗼𝗿 𝘁𝗵𝗲 𝗱𝗮𝘆 𝘆𝗼𝘂 𝗲𝗻𝘁𝗲𝗿 𝗧𝗵𝗮𝗶𝗹𝗮𝗻𝗱 𝘄𝗶𝘁𝗵 𝗶𝘁?

a single entry 90 day Non-O visa is valid for 3 months from the date you get it issued, so you have 3 months to ENTER thailand on it and when you do you will get stamped in for 90 days

𝟯) 𝗗𝗼𝗲𝘀 𝗮𝗻𝘆𝗼𝗻𝗲 𝗸𝗻𝗼𝘄 𝘄𝗵𝗮𝘁 𝗱𝗼𝗰𝘂𝗺𝗲𝗻𝘁𝘀 𝗮𝗿𝗲 𝗿𝗲𝗾𝘂𝗶𝗿𝗲𝗱 𝗳𝗼𝗿 𝗻𝘂𝗺𝗯𝗲𝗿 𝟱 𝗶𝗻 𝘁𝗵𝗲 𝗽𝗶𝗰𝘁𝘂𝗿𝗲 𝗯𝗲𝗹𝗼𝘄?

It is a letter YOU write stating you are the parent of a thai child and want to get a Non-O visa issued based on that