I should be able to answer your questions. I recently finished my validation with the Thai Flying Club. You will have to join the TFC, and have them do it for you. You cannot do the process on your own. You will have to sit an air law exam at the DCA in Bangkok and go through a background check. The whole process will take at least a month or so. No need for a Thai medical for a validation.
Other answers:
2 - Are the VFR charts up to date, accurate and readily available?
Not really. The only real VFR charts I have found are WAC charts which haven't been updated. There are some "homemade" charts on the TFC website that seem to work.
3 - Are there a decent number of airports and strips that are suitable, well maintained and generally with low to moderate pucker factor?
I really haven't ventured too far from Chonburi yet (where the TFC is located). There are a lot of small airports but most are unimproved and I wouldn't go there without someone else who has already been there with me. Commercial airports would be no problem though. There are some youtube videos of the pattern at Chonburi which is pretty interesting by itself.
4 - Are there a decent number of servicable navaids?
So so. But the radios in the club aircraft are average. Highly recommend a GPS if leaving the airport area.
5 - Is the Queens English spoken on the radio?
A form of english will be available. Can be difficult to understand.
6 - How usable are the systems for getting met information, NOTAMs etc for a low-houred non Thai speaking pilot?
I would find a Thai pilot from the club to help you with this. The manager there is a great guy and always willing to help.
The TFC is pretty much the only option you will have. Thailand's GA rules are archaic to say the least, and there are not many rental options.