if several opaque object3d are added to a world in particular order, can i assume they are rendered in that order?
I don't think it works that way... I thought there would be some depth checking for the Object3D's when it comes to rendering order
Depth is only relevant for translucent objects. Anyway, it doesn't work for opaque either, because they are sorted by state to minimize state changes. So: No, you can't assume that.
here's another question: in a World, are transparent objects rendered after (speaking of time) all opaque objects?
Yes, of course. Otherwise, the pixels from the opaque objects won't shine trough but overdraw them.