, the decision will be based on their travel history too, have they already been in Thailand a while ? If refused, their ID's could be flagged on the system and getting in COULD be an issue at any border, land or air.
The BE stopped because the letter no longer complied with the degree of proof required by the Thai authorities. Reading between the lines, the BE wasn't in a position to check what people were presenting.
The form also says that the evidence provided should be simple, easy to understand and clearly show the key financial amounts etc. As I said above proving you have the pension pot is easy, proving the required INCOME is not as you can vary your drawdown as you wish, which is exactly what the OP is doing.
As you are drawing down rather than getting a regular pension or annuity income I think you may be in a difficult position getting the required documention. You can prove how much is in you pension pot with ease, but showing the monthly income could be a problem.
In the UK, it is much cheaper to get a single entry tourist visa (£25) that gives you 60 days on entry rather than the extension (£43), also it saves having to waste any time going to immigration.