When you say it closes, what actually occurs,? If your applying for a non O based on retirement, its design is for someone who wants to retire here and will extend to the full year during that 90 days, and therefore an exit date isnt really required.
if it mentions BOI, that sounds like your looking at an LTR visa, opposed to a normal non O based on retirement. The LTR is expensive and has high qualifying factors, but in the long term a better visa, hence the cost.
for the dtv and the tourist visa when making the application, yes the flight into Thailand must be filed as leaving from the country in which the embassy application is made. But as Brandon said, you don't have to stick to the info you supplied for the physical entry. So you can enter on exempt, then receive your dtv, and at your leisure (within the 60 days of your exempt) pop over a boarder and return with your pdf visa to activate it. Or even leave Thailand in 60 days travel the world for a year and come into Thailand anytime in next 5 years (as long as hou havent lost your pdf) to activate it. The downside of coming when the dtv is still pending is they may ask for more info that you can only supply from the UK, eg copy of p60, copy of utility bill, etc.
400k is the absolute minimum at any point in the qualifying period, you need it to be say 420k, and then on the day you can do a withdrawl of 10k without any worry and enough to cover any bank charges that might pop up during the book refresh.
Technically you can stay out until the day before your visa expires, it will be mentioned in the reentry stamp. However, unless you have written permision from your school saying how long they are allowing you to skip lessons and a good idea would be the why, you could be refused entry. So if they give you 3 weeks in the letter, you have to return within the 3 weeks.
but you also have to consider in the worst case scenario, that if you get a remittence of 800k from an agent in the year you become a tax resident, you are liable for the tax on that for the next 5 years.
he means that if you know your moving next year (and will be tax resident next year), any money transfered this year to Thailand is not treated as assesable if this year you spent less than 180 days in Thailand.
Presumably if on an extension you will have a current work permit to work in Thailand. This needs cancelling and returning to DWP before you leave and cancellation of business visa at IMO. You need supporting document stating last day of work. You will probably have to request a denial of extension stamp giving you 7 days to leave Thailand, depending on your timeline. Expect extra scrutiny in the embassy in LAOS on your paperwork, as to the sudden change in your circumstances, one day working in Thailand, now not going to work in Thailand but having an unbroken income stream.