Shepherdless your presentation of the figures is very good and you obviously know what you are doing. My comments are intended to be supportive.
I wonder if you've come across a book called Sects and Stats by James R. Lewis. He's a religious studies scholar who uses census data to study small sects in anglophone countries. So very similar territory, except he doesn't discuss JWs. But his work may be helpful in terms of methodology.
https://www.equinoxpub.com/home/sects-stats/
When I read this book I was initially excited about applying his methods to JWs - until he said that the data are very expensive to access.
Also I suppose you know of the work of Ryan Cragun and Ronald Lawson on JWs, Mormons and Adventists.
https://www.ryantcragun.com/the-secular-transition-the-worldwide-growth-of-mormons-jehovahs-witnesses-and-seventh-day-adventists/
Cragun is good with stats on Mormons in particular, and responsive to emails.
Plus there is that essay by David Voas on JWs, I will send to you if/when I find that book again!
https://www.jehovahs-witness.com/topic/172862/british-sociologist-predicts-possible-collapse-jehovahs-witness-numbers
In fact David Voas is a very interesting sociologist it may be worth you following up, because he supports the secularisation thesis in a particular form that is supportive of some comments you've made on the subject.
Years ago I had a conversation with scociologist Steve Bruce about religious decline and JWs in particular. He said that his latest work (this was around 2006) was in collaboration with David Voas and they were working with evidence that the mechanism of religious decline is largely the failure to transmit religious affiliation from one generation to the next. In other words, people who are accustomed to attending church/KH tend to keep on doing so. Religious decline is not usually a result of people leaving church en masse. Rather what happens is that, at a certain juncture, the children of the devout are not fully socialised in the church, so they don't continue affiliation as adults. Decline therefore results as the devout age and die, and their children don't replace them in the pews/KHs.
This may seem obvious actually, but it's the subject of much debate among sociologists. In fact even the very fact of religious decline is (amazingly) in dispute among some prominent sociologists.
Another excellent book on religous decline is Religion and Demographic Revolution: Women and Secularization in Canada, Ireland, UK and USA since the 1960s by historian Callum Brown.
https://www.amazon.co.uk/Religion-Demographic-Revolution-Secularisation-Religious/dp/1843837927/
He argues for a cultural and gendered explanation of religious decline.