UK is a bad example, you do not have to be tax resident anywhere if from the uk. Yes if you are in the uk to long or have to many ties to it them you may have to pay tax. But most people in Thailand won't have to, unless going back there to only stay 179 days in Thailand.
I applied in Manila at the start of the year, excluding 2 days waiting for documents from me, it took 3 days of processing. Back then I think there website may of said 5 business days as I was expecting to be gone by the following weekend (had my flight booked) after arriving on a Monday
not everyone is required to pay taxes somewhere. The uk for example you don't have to be paying taxes to anther country to become a non tax resident, then just make sure you don't make yourself resident in another country like spending less than 180 days in Thailand for example
I'm no expert, but I think being employed by a thai company is going to be a problem. I'm sure someone will be alone soon that knows better
Edit.
After reading your post again, I take it you won't be mentioning the Thai company when applying so probably all be fine. Will your bank statement show thai income?
The guy posting this is my friend, like me he has 2 uk passports as he works in the oil industry. The reason he is allowed 2 passports is because he will often need a visa from the country that he will be working in next, but will also need his passport to go to his current work in a different country. So he is allowed 2 passports so one can be getting a visa whilst he is using the other one. It's not a new and an old passport. They are both valid.
Unfortunately he has used the passport with his thai DTV in for a visa to go to India and it won't be returned for 2 weeks.
What will happen if he enters thailand with his other uk passport visa exempt( the one that isn't associated with his DTV) will that effectively cancel his DTV visa?
Can he show a copy of his uk passport associated with the dtv visa and enter on the passport that he has in his possession?
I know he will not need more than the 60 days that you get stamped in for with visa waiver, but it's about what will happen to the DTV will it become invalid or still be able to be used on his next entry?