What are the requirements and process for obtaining a certification letter from Bangkok Bank for immigration purposes in Thailand?

There seems to be some confusion. You seem to be talking of bank statements and the bank letter. With Bangkok Bank you apply for the yearly statement 7 days in advance. When you pick your statement up you then request your letter and update bank book. Then with all this done go to immigration and request extension. This is how I do it in Nong Khai and immigration seems happy.

If I understand your situation you will be applying for another extension of stay based on retirement. In which case you will require 12 months of statements, I usually request 13 months as they need to go back to the date you got your current extension, and with Bangkok Bank you need to order them 5 working days in advance. For me I pop in and order them and when they arrive I pick them up, ask for the guarantee letter for immigration, update my bank book and head to immigration. Only the bank book update actually needs done on the day, the rest can be done up to 7 days in advance. You should be able to use any branch.

Best advice is to go to the IO and ask for the forms and a list of requirements as they differ for each office .My bank letter was valid for 7 days but some offices say only on the day so you need to check locally

The letter is good for as long as the Immigration Office you are using will accept it, ask them not an agent ;)