no one can give you a definitive answer on that. Each time you enter will be different & each IO will be different too. It's unlikely but there is still a possibility of an IO taking issue with it.
DTV for 5 years on soft power but you will still need to work something out for the last 3 years to get you to 50, you can get the platinum privilege (10 years 1.5m baht unless its changed) or LTR (however requirements are steep for the LTR)
Yes but make sure you keep to the terms of your current extension (not dropping below 400k) and personally if it was me, I would also make sure the 800k is there before you get the extension just incase something is wrong or something goes wrong.