Not all income is assessed. Only assessable income. You are inferring in that sentence all income will be assessed if you are a tax resident. It is not. Are you a Thai Tax Resident and have you ever submitted a tax return in Thailand?
Immigration did NOT set the rules for the DTV. It was the MFA. Immigration do have final say on right of entry though even with a visa. The MFA website clearly states that.
Still cheaper than I thought as I am sure 400 SGD (10k THB) was always mentioned on that route and that was over 10 years ago. Most went to Phuket or Bangkok as it was cheaper.
Not much then. I just remember my boss in Singapore complaining about flight prices one weekend when he took his family for a long weekend. He reckoned he could have gone to Japan cheaper for a holiday.