Always worth checking their work before it goes off to the MFA. My affirmation of freedom to marry was incorrect (they put that I have no children from a previous marriage, which is probably the 'standard' line) and my affirmation was rejected by the MFA supervisor's office. Luckily I took it there myself so I was able to sort it out, but that did add a few hours to the whole thing!
Obviously you'll need someone who can read Thai, and English, in order to do that :)
says, or a ticket out of Thailand before 30 days is up on your stamp. If you don't get on the plane you can't extend anything, can you? Standing in front of check-in staff who've already decided you're not boarding telling them you'll do a border bounce won't be a great start to your trip.
That's exactly my point. The airline staff at check-in will look for a visa in your passport that covers the whole trip. If you haven't got one, then you will need to show them proof of an outward flight that leaves Thailand less than 30 days after you arrive, or they won't let you on the plane.
Even a single entry (60 days) might not be enough. I know you can extend it and cover the whole trip, but the initial entry will be for 60 days, and all it takes is for the check-in staff to check their list of visa requirements, see it says 60 days, decide you're not boarding, and you have a nightmare scenario right there at the airport trying to explain how you will extend it once you get there with the check-in staff just looking at their screen which tells them your visa doesn't cover the length of your trip.......no trip. For the sake of £40 I'd buy a one way ticket Bangkok-Kuala Lumpur and show them that.
If your return flight out of Thailand is 86 days after you arrive, and you have no visa in your passport, you may have problems checking in to your flight. If your return flight is with them, they will see your return date and they will look for a visa that covers you for the entirety of your stay. If your return flight is not with them, and you essentially have a one-way ticket with them, they will also look for a visa in your passport, or proof of an outward flight 30 days or less after your arrival in Thailand.
The airline will have the responsibility (and cost) of flying you back home if immigration deny you entry at the Thai border, so they might not let you board the plane in the first place.
Best advice I can give you is to have a booked flight out of Thailand 29 days after you arrive (unless you are able to get some kind of Non-Imm visa which would allow a 90 day stay) and once you arrive in Thailand you can do all the extending of your 30 day visa exempt stamp that other people have advised you about.
You don't need to use the flight out, but you may need to show it to the check-in staff on your initial flight to Thailand.