The Non-B makes sense as that is for working inside Thailand. I can't see other visas being tied to immigration as there are just too many variables for immigration to get their heads round.
SS is covered for a US person but that doesn't mean all pensions are. Most of the DTAs do cover pensions earned in government service so civil service, military etc
If you are on an extension of stay then that extension finished with your contract, you should have cancelled both your work permit and the extension on your last day of work and left the country.
If you are on an actual visa (check your passport) you can stay until the visa expires
I've been here 8 years on long term extensions and last I checked they won't give me one. I'm not married and don't have any property though which is what counts me out, at least at my amphoe. To be honest I only tried to get the book in order to get the pink ID card but I find a driving licence is just as good in most situations
You can show money in your Australian bank when applying for the 90 day Non-O visa inside Australia but to get the 12 month extension inside Thailand you will need to show funds in a Thai bank, seasoned for 2 months before you apply
if you want to be pernickety you can go through all 60 odd dual tax agreements and find different exemptions in each one. But the basic principle that income is taxable whether or not you are retired. It applies to income earned inside Thailand and it applies to assessable income remitted to Thailand from elsewhere
credit is not taxed but it matters how you pay off that credit. Say you use a credit card for all your spending in Thailand and every month you clear the balance on your credit card using money in a bank account which you have had paid in there from monthly income. Then you are in effect remitting that income and would be due tax on it. Obviously if you clear the balance in some other way not involving income that doesn't apply