Amazing information. You highlighted information I had not thought of.
It is a good point that inactive ones return when having children. Sadly, I have seen that is often the case with ones that leave and marry unbelievers and then return upon childbirth.
There is regularly discussion around Watchtower collapsing suddenly, which is quite unrealistic. Older ones don't leave, they die out. Your analysis of the median age shows what will likely happen. The religion will slowly decline as the members age and die, with fewer new recruits to replace them. The 5 year increase in median age over a ten year period is quite astounding. Pity not all countries have reached this stage yet.