Unless you are doing the 90 day report where you did your previous report or extension of stay you will have to formally change your address to the hotel where you staying.
You appear to be in Ubonratchaithani.
The immigration office in Phibun is no longer open. There is a office in Ubon city at Rajababhat University now.
You cannot get a extension at a border crossing.
That is form used for permanent residency.
The correct form can be found near the top end of this pinned topic.
https://aseannow.com/topic/631223-immigration-templates/
You will use both passports when entering the country.
Immigration doesn't require letter a letter when the passport is issued outside the country.
They will transfer the stamps without it.
You will need a letter from your employer stating the last day you will be working for them.
You will need to apply for the new extension on or before the date on the letter.
There is no automatic extension. You have to apply for a extension that will be denied and 7 day days are granted to leave the country and the 1900 baht fee is paid.