Key discovery - the Outlet Blend Door 'drift' problem I had was because I had assumed that blend door was a 360 degree engagement that just needed to be set right at the start. However, after disassembling the factory gears it now appears that there's what looks like a 230 degree limited sweep with a tab on one gear that hits a limit. So while yes the gears need to be meshed correctly at the start, the Controller finds it's position by pushing to one of the limits until it can't move any more, and then coming back. So it was always pushing forward that bit too much when unlimited, and coming back less than it should - hence the drift.
Suffice to say, the custom setup I created does not have this feature. I'll have to go back and work out whether this is still possible in the space I have and if I can implement. If I'm still OK, I'll replicate the factory gears first. Then I will work out how to shift the mesh tabs and limit tabs to have the blend door still face the right direction even though my actuator will be driving from a different position.
Lukeyson