not true. The regular tourist visa is only up to 90 days. The STV is up to 270 days. Yes every country can apply for the STV if it’s available via their embassy. But can only be issued by passport country.
I haven’t heard of any backtracks from the Thai government. It may be that your embassy is just not offering it yet. It was announced a couple of weeks ago that it was available for every country world wide. It’s up to the embassy to update their website and to start accepting applications. I’m sure there was some people in the past week or 2 that have applied for it (at least it was discussed in some fb groups)
At this point it cannot be changed in country. If you cancel the STV and leave the country to apply for a new visa upon re-entry you’ll have to quarantine in an asq.
So if you qualify for the retirement visa I’d say do it now before you come and arrive using that visa not the STV.
From everything I’ve read the STV cannot be changed to any other type of visa while in country. But who knows what will happen in 9 months (if you do the extensions).