You will have the difficult task to prove to the UK authorities that your wife has exceelent reasojs to exit UK at the end of her visa. eg, good job in Thailand (letter from employer stating the period of her leave and the lenght of her employment), own house in Thailand (or lomg term rental agreement), need to return to look after child, parent or close relative, proof of ealier travel showing abiding by visa conditions. Difficult for a Thai wife to get UK/US/Australian visa. All have similar requirements.
Provided that you can obtain the required medical certificate from a doctor/,edical clinix/hospital and the residency certificate from your local Immigration office.
If you are not a tax resident of Australia then you do not receive the tax free threshold and pay a minimum of 44 cents in the $ tax. Much more than in Thailand. And the tax authorities in Thailand do not know how much you are taxed in Australia, So I expect that Thailand will tax you at 15% or at 0.
First, it depends on your country of passport. If you paid tax on that pension in your home counrty and that country has a dual tax agreement with Thailand (like USA, Great Britain, Australia, New ZSealand and many other countries) then it will not be subject to Taxation by Thailand.
Before you can get a maariage visa (from a Thai Embassy/consulate) outside of Thailand or an extension of stay based on being married to a Thai (from an Immigration office inside of Thailand) you need to have your Thai marriage recognised by Thai authouities. Getting married in Thailand is a complicated process. The first step is to get a letter from your embassy stating that you are free to marry.