I do not use the lump sum approach, I submit for marriage to Thai visa extension the proof of 40,000 monthly option. The 100,00 amount is what I also have to show in my bank book, nothing to do with visa requirements, just one of the extra items that my office require which is not quoted on the requirements needs. Much like every office has its own quirky rules. When they get a new OIC, he/she brings in their own supplementary rules. You just have to find out these and apply them, no good arguing that they are not on the requirements list
Every Office is different, for example in Hua HIn where I do my extension, I have to produce my bank book with 100,000 B in it, and the witness has to attend.
Every office want something different, the only way is go and ask what they want before you start putting the application together, make sure they stamp and sign the list. Make sure you take that list with you, why you go to extend.
Remember that an application for extension by marriage to Thai has to be approved by Bangkok, therefore it needs to be forwarded to them at least 4 weeks prior to renewal date, meaning the application date must be a month in advance
Every office has its own requirements, in addition to the laid down requirements for instance I have to have a Thai witness to my application, and I have to submit his/her details and copy his house book. Additionally I have to produce a bank book with 100,000 baht in it, which I am informed is increasing to 200,000B this year. So my advice pay them a visit and ask for all details prior to application
Always check with the Immigration Office where you are completing your extension for requirements. Pay a visit and ask saves many other visits. Every Office has its own quirky rules. For instance, where I do mine, they require to see my bank book showing minimum 100,000
Have extended by this method many times now, sometimes they visit other times not, last time they did not require the photographs, everywhere different, Here i am required to show my bank book showing at least 100,000B in it, There nothing about this on the requirements
I have been extending my visa via the monthly income system for years now. My Thai bank print off the monthly income on individual sheets for each month and I collect them from the bank having first arranged it with the bank, then take them with me when I visit the office for my extension