Yes you can do that but you will need to honour the terms of the extension based on 800k for the full period of the extension. When it comes to the next extension you will need to show 12 months of statements from both banks. Bangkok Bank to show you honoured the terms of the extension based on 800k and Kasikorn to show you have 12 months of transfers from overseas of a minimum of 65k baht every month for the previous 12 months
Did you explain to the immigration officer your plan to leave Thailand and what dates you would be out of the country when you applied for the extension so they could time any home visit for when you are there? If not you should go back to the relevant section and explain to them. Don't rely on the fact a different section gave you a re-entry permit
Yes this will be a problem. But if the move is essential for some reason go to the immigration office and explain, taking a new map if you had to supply one and see what they say. Just a TM30 will not suffice
It depends on where you apply. Some places want 3 months of statements but as long as the final balance is 500k baht its ok, others want to see 500k baht for the full 3 months
SerenePineapple9021 might work, might not work. If you're happy to throw away another application fee go ahead, but it might be worth trying to work out why you were refused first