Is there some way to control the center and radius of rotation in the Orbit function?

It should be the geometric center of the model, but it will change if you walk or mouse zoom. Turntable orbit will also change it by pressing <CTRL>+<Left-Click>. It resets to the center when you press X/Y/Z/<F10> or push the corresponding buttons. When you finish walking, it’s set to where your “feet” would be. Mouse zoom tries to zoom on whatever is under the mouse, and that becomes the center of orbit.

The "Zoom to Selection"-button also sets the center of rotation to the selected element (5.20.3 version)

regards
Jon