So 2 things, the "60 day covid unable to travel extension" is NOT an extension of your tourist visa, non-b or non-O. It's it's own separate extension of stay. Your tourist visa is gone once you've transitioned to that.
Because of that, the 60 day unable to travel extension starts from the day you apply for it, not added to the end of your current "visa or extension" since you're not actually extending anything, you're starting a new extension of stay.
I don't doubt that the immigration officer told you incorrectly, he probably didn't understand it either since it doesn't make great sense to being with.
Visa status doesn't change the quarantine requirements. You don't need a vaccine if you're going to do ASQ in Bangkok. You must have a vaccine to do the Phuket sandbox though, no exceptions (including Thai people).
There is no nationality restrictions on the Elite visa.