Anonymous participant 0 baht to open account is quite rare because the banks basically "require" to purchase an insurance policy. Without the Thai account you end up paying transfer fees or ATM fees and then you will end up paying more than the agent anyway. Plus the time of going to dozens on bank locations is worth something unless you have nothing else to do.
When I opened mine a few months ago, the bank insurance was included in the agent fees. So the difference between trying to do it myself over many days or getting it done in less than 2 hours was only 3500 baht. I preferred spending those several days I saved to do other things
90 day reports are for basically any stay in Thailand of 90 days irregardless of which visa. But no, you don't have to supply the same information as when you get the DTV.
The 90 day report is simple with a few pieces of paperwork. The purpose is to update immigration with where you are living and is not related to the visa.
You can stay up to 180 days (it doesn't matter how many), then leave the country and return and get a new 180 stamp each time. Easy, I've already done it.
Extension- still unknown what the requirements will be
1) It depends on which embassy you apply through. Some require 6 months, some only current account balance.
2) First point to bring up is extension versus reentry. No one has extended yet and so not sure what will be required for an extension. If you leave the country and re-enter then nothing is required. Every time you enter you get 180 day stamp. (I have already done this)