It is the airlines responsibility to insure you have a outward departure from the country you are going to. If immigration on arrival does not let you enter the country than it is the airline that is responsible to fly you back. The airline can be subject to fines or disciplinary actions if passengers are not screened properly upon checking. The only exception is if you have nationality or long term visa to allow you to stay.
when you open an account you are given a bank book which when updated shows all transactions on your account. It has your name printed on the first page. If you loose it or it is stolen you have to make a police report before you can get another one You will need it to renew your retirement visa plus photocopies.
It is not bank statements you need but your bank book. It has to be updated for the past 12 months including up to the day you apply for your renewal. Plus a letter from you bank which is valid for 3 working days after the bank gives you the letter. The 800,000 balance has to be in your bank 2 months before and 3 months after you apply for renewal. Minimum balance can not go below 400,000.
Any branch of Bangkok Bank will give you a letter verifying you bank details. The cost is 100 THB and you will have the letter while you wait. Usually less than 10 minute wait.