if you bring it into Thailand while a Thai tax resident then Thai tax rules apply and you will have no UK tax to offset against Thai tax. Basically the same as above
indeed, but there may come a time when immigration may deny entry. And also there is the requirement to leave Thailand every 90 days and stay out of Thailand until you receive the visa
Yes it is if you are a Thai tax resident when you bring it in. The lump sum is tax free in the UK but is viewed as income in Thailand and Thai tax is applicable plus you have zero UK tax to offset against it.