I appreciate your thinking and time you've spent on this bohm but I think the above is over egging the pudding.
The number of baptised witnesses is never published. The figures that are used to calculate growth rates are for publishers. Publishers are not all baptised.
Also, the figures always are based on peak publishers. They are massaging peaks through things like having 30 hour pioneer opportunities when the CO is coming.
As you say, baptisms happen at CAs as well as RCs.
Trying to get these extrapolations is IHMO a little bit of a waste of time as the variables are too great. I think the overall trend analysis of the YB presents a more useful indication of the actual health of the organisation. To that end it's quite simple....
More and more time is being spent converting less and less people.