I've had a few 60 day visa's before, but the London embassy have never asked to see evidence of a return flight because that's not required. The airline may ask for evidence before they let you fly (although you don't need one) but that's a different matter.
I had that. It was very difficult and expensive to extend a policy when already abroad. You just have to bite the bullet. No good lying online as to where you are, as you won't be covered...of course. Not an answer for your query but when I entered on a 60 day visa last time, I bought a 90 day policy as the extra cost was minimal in case I stayed longer.
An onward ticket or a return ticket have the same effect. Immigration aren't bothered where you go afterwards. Don't give out partial or confusing advice. It helps no one.
If you live in Bangkok, maybe. If you don't it sounds like major aggravation for little return or a huge loss. Stick your money in a bank, earn some interest and have no hassle.
I have entered with a 60 day TR 3 times (with a paper print of the visa) and each time I carefully check what they have stamped before I move away from the immigration counter.