It is fine to disagree, I enjoy a well thought out discussion.
I just think focussing on the radius is misleading no matter how many people may do it on the internet.
I agree entirely about the radius changing, especially when measured from the centre of the wheel to the floor, as when the tyre is inflated it increases initially as the flat spot lessens and if you inflate to silly/dangerous pressure the tyre would balloon across its tread width too.
But you would get to a true radius of a circle instead of a fictional radius of a circle with a flat spot.
So I agree that measured radius changes and therefore have to agree with you that calculated circumference changes.
I do make the assumption that a steel belted tyres construction does not allow the fabric to stretch and that the radius is only changing because the flat spot at the bottom due to flex under load and inflation changes.
Therefore as no material is removed or added to the tyre as it is inflated the true circumference, the distance it rolls forward in one revolution remains the same. And that is the measurement that should really be used in calculations.
The more calculations you use to get to that point like wheel size, width aspect ratio the more errors creep in.
I think it would be great if the manufacturers actually published the circumference as measured, but I imagine there are not enough people swapping sizes to warrant it and diameter lets people quickly measure what it looks like in the wheel arch so that is what seems to get used.
Next time I have a wheel off I may just roll it along the ground and measure then put it on the car with 10psi in it and again with 40psi but I honestly cannot see it being any different the higher inflation just minimises the flat spot at the bottom and therefore lessens the flexing of the tyre on each revolution and therefore the heat generated in it.
I did have the same issue as you in not being able to find a comparison to cross ply tyres, but I have in the back of my head from years ago that cross plies were 88% aspect ratio and that an unlabelled radial was 80% I went up to 15" wheels on that basis with a common (cheap for what they are) size 195 50 15s and still have a smaller circumference than the tyres I took off.
The other car is still on crossplies (only fit for keeping it off the floor) which are massive in comparison.