I think your extension will only run to your expiry date. If you extend in April then you've only got 6 months. Get a new passport here in Thailand around Feb 26 (allowing delivery time) and apply for your new extension in April on the new one.
Get the proper multi-entry visa, it's not expensive. I think you can only get it in London now. As far as I know Glasgow and Hull don't do it any more. Other option is to get a letter from your company saying you need a second passport. Alternate passports between trips. I'm assuming you're British.
Two weeks ago a friend of mine went into his branch to do a transaction. He was told he couldn't as his account was frozen. He was taken to one side by the manager and informed that his account had been opened using a visa that was no longer acceptable. Luckily for him he had applied for a long term extension only a week before. He showed the manager the 'under consideration' stamp in his passport. They unfroze his account but he has to go back and show them his proper visa once he gets it approved and stamped in his passport.
I am suggesting that if you come here with three kids to support, $100k and only a TEFL teacher's pay to go on then you are in for a fright if you think you will live well.