I was primarily addressing your claim 'no border bounce by sea'. Not only at Ranong, there's also Koh Lipi to Langkawi, I'm sure there are loads more...
most embassy won't issue the letter. With no 90 day report in the system at Bangkok, you options are use an agent or head to Pattaya and use an agent there, to help with residence cert etc
Otherwise try the head office branch of Bangkok bank and find out exactly what hoops they want you to jump. They wanted an embassy certified copy of my passport. Which is why it's easier to use an agent....