The Korat dialect is VERY different from other Isaan dialects. What you will find is that people from different parts of the north-east will find common ground in their languages. Many people speak more than one dialect due to the mixed nature of the population and overlap of language. My wife is from Korat, our neighbours speak a variety of languages from Lao (northern) Northern Thai and a smattering of various Lao-Isaan dialects. When my wife and her sister start talking their Korat dialect, no-one understands (which is probably why they do it:) )

Everyone I speak to in Central Thai understands, and is able to respond so that I do too. Some of the older folk might have to pause a bit to think it through, but they still get it in the end.

It's not just the official Thai language, it's the one that people from all the different language backgrounds use to speak with each other, unless they have another common dialect, or know enough of each other's dialect to get by. Thai people from the regions away from the central area are often multi-lingual. They enjoy having fun with many different aspects of the multiple languages found in Thailand. As learners of Thai, we would need decades to catch up with the average rice farmer in terms of their language skills. Central Thai is the one that we will find to be most useful in any part of the Kingdom. That's why they teach it to us in schools.

When you enter the country you get an 'admitted until' (with a date) stamp. That's how long you are permitted to stay. With this visa, as others have said, that will be 90 days.

Along with the pre-paid envelopes for the Legalisation office, and for the Thai Embassy to send it on. Looks like you might need to send it to someone in the UK who could get the stamps and postal orders for you.

The Postal Order payment might be an issue if you're not in the UK.

The paperwork is all yours anyway, which you have to get yourself, so you might as well do the travelling about as well. Why pay someone else to do it for you? The immigration offices will even tell you what you need to bring!

The optimism you see in that is, perhaps, questionable seeing as the extension lasts for a year :)

Get the letter translated. They probably think that they're helping by giving the letter in Chinese, seeing as they're in China, they may assume it's for the Chinese authorities.

It’s not that bad of a thing for them. They don’t run the risk of being arrested at a police check point/raid and deported. Just because they’re too lazy to do the paperwork doesn’t mean that someone should feel that it’s ‘ok’ to run that risk. It’s probably unlikely that the worst would happen, but if it did saying “but the immigration officer told me to do it” might not help you if you’re faced with a trip to the IDC.

There’s no scan, no check of her passport other than by the airline staff.

They try. When my wife visits the UK they stamp her in (a record of that is kept) but when she leaves, she just leaves (no stamp, no record of her having left) so they don't really know if she left or not.