Cheapest is NT Telecom with unlimited max speed for 6 months for 599B but this is data only (no calls) and coverage is not as good as AIS. I have used it as an internet backup as it really is unlimited and can tether. I get around 30Mbps down, but only 5 up at home in Chiang Mai.
it's that you can type it right and it then changes it to wrong if it misreads your passport. It's not a great design, it should take the passport image first and then ask you to check the data. Not have you enter the data and then overwrite it. I can understand people missing this and this is not the first case, others have been caught like this as well, they entered data right but the system overwrote it wrong.
it's there, check again. AFAIK it's an obligatory field and should be there on any passport. Second row, position 22-27. Possibly you had entered it right but then it misread from the MRV.
you realise that says that pensions are taxable in Thailand if the person is resident in Thailand (i.e. over 180 days per year). That state refers to the state of residence, read it carefully.