It seems that at times I'm able to see editable boundaries of the render region to make a small adjust to min/max X/Y and see corner grabs to adjust an X-Y boundary. At other times I cannot get these and have to redraw the Render Region. How do I get these adjusts to appear/disappear?
You might want the render_border operator. First switch into camera-mode with Numpad 0, then you can either access this operator by the shortcut CTRL+B (setting a render border) and CTRL+ALT+B (resetting the render border to the whole frame), or by opening the search bar and typing Render Region into it.