Yeah, it gets crazy. I think RC is X-up (right-handed) and Zbrush is Y-up (left-handed).
For number 2, I can confirm that when creating a mesh on blender (a cone in this example), and then import it to Zbrush as an fbx format - it reads the correct orientation. I mean, the Z-up axis of a cone in blender will be “converted correctly” / yes it flips, as a Y-axis in Zbrush. The cone will be sitting on the ground plane as shown on the screenshot below, thus zbrush fbx import is able to properly see the mesh’s up-axis.
And it’s the same thing, between RC and Zbrush, even though RC’s using a different up axis from Zbrush’s Y-up axis, Zbrush was able to properly orient the model so that it will be sitting flat on the ground plane (Y-axis).
I think the problem in here, is the different orientation between the two programs, specifically RC’s right hand and Zbrush’s left hand orientation. When the two programs are exporting their fbx models, it will “flip” the model based on its up axis and since they are using a different hand orientation and up axis, the model’s rotation will never matched. That is why, my current workaround is to open the fbx on blender, then apply (reset) all the rotation values, before importing it in RC.
Below is a sample screenshot of an fbx model that is export in RC and Zbrush, and then imported to blender. We can see on the transform properties that the models were rotated on different axes - RC model was rotated -90 on Z axis while Zbrush model is 90 on X. This is probably the reason why the orientation doesn’t match when importing the fbx model back to RC. Both program’s axis orientation is from north to south, a total mess.
The way I see it, RC should have an option to select the up-axis on model import and export as RC’s axis orientation is very uncommon compare to other 3D programs.