1. There are a couple of things, I don't remember exactly which apart from smoke, that escaped the generic thing I did to pick up most of the things that changed. I had to do some hacks to get those working.
2. The main problem with 3D geometry is that it doesn't always correspond with the real-life geometry of the objects, so pixels get a wrong z value. This caused overhead wires to be visible through the bridge above. Front images are visible through the back images on the tile in front. Vehicles disappear partially into the ground on slopes, and also come into conflict with front images, as well as the back images in front of these again.
I therefore think it's time to try out plain 2D billboarding (so more or less like my previous attempt, just divided into semi-static batches rather than fully streamed), but the trouble is coming up with good z-values that "always" work. Simutrans doesn't simply render things back to front, because that won't work well at all, nor does it get it 100 % correct the way it does it either.