(1) What are these inner_x, inner_y, inner_w, inner_h parameters in particular?
Those are the parameters of the "inner" window. That is, the contents. The "outer" window is what contains the inner window as well as all the decoration.
(2) Not knowing any better I chose 'AppleWMFrameClassBorderless' as this seemed most appropriate to denote the CoreGraphics not to put any of the usual Aqua title-bars, drag-knobs, its own border, etc around the final result. In fact, what does this 'class' parameter like 'AppleWMFrameClassDocument' denote, regarding the shadow drawing, if it affects anything at all?
I think it should have no shadow. That's used for menus, tooltips, splash screens, etc.
(3) The second, the 'attribute' parameter seems clear: it looks like only 'AppleWMFrameActive' and '0' make sense here... ?
Yeah, that doesn't matter if there's no frame.
(4) Most important unresolved issue is termination: how to withdraw the shadows by means of Apple-WM extension? In particular, if quartz-wm is started manually, and terminated by Ctrl-C, then it withdraws all shadows. How does it do that? :-)
it unmaps the window and reparents it to the root window.
In fact I would put this code to be executed on 'UnmapNotify' events.
Yeah. I didn't look at the code, but if you still need help, I'll be able to look at it on Monday.