I'm obviously not making myself clear, sorry about that.
Previews are Adobe RGB, no argument. Previews are created when you first view an image in Library Module (or first time after the image has been edited in Develop Module). They are cached and stored in Lightroom's preview cache.
But I don't think they are used to display the image in Develop Module. Develop Module (I think) renders the image from the ProPhoto RGB working space image (you agree that Lightroom uses ProPhoto RGB as its working space?).
That's why I don't think LR needs to update the previews after each edit operation. Because it's not using them at that point. It doesn't need the preview until next time you go to Library Module (or any module other than Develop). In Develop Module it updates the display in real time, obviously, but not the preview. I think it's rendering the ProPhoto RGB working space image data directly to the display (using colour management, obviously, mapping to the monitor colour space by means of the image profile and monitor profile in the usual way).
Library Module (and other modules except Develop): LR renders the cached Adobe RGB jpeg preview image.
Develop Module: LR renders directly from the ProPhoto RGB image data.
You may not agree with me, but I hope I'm making myself clear. And I'll be quick to own up to my error if anyone from Adobe tells me so. Or if you can explain why the image rendered in Develop Module is visibly different to the image rendered in Library Loupe view, if they're both using the same preview image!
I'm trying to find references about the differences in rendering between Develop and Library modules. I've found quite a number discussing that the image as rendered in Develop isn't always the same as the image rendered in Library. I infer from this that Develop doesn't simply render the preview, or the image would
look the same. See for example: http://forums.adobe.com/message/3498280#3498280
PS - edited to add:
To illustrate what I mean: here are two screen captures of 100% saturated colours in a ProPhoto RGB image. The first is a screen shot of the view in Develop Module, the second is the view in Library Module. Obviously the colour spaces are mangled in the screen capture, but the point is: the images are different. The screen shot in Develop Module has more saturated colours, which you'd expect if Develop Module is displaying in ProPhoto RGB while the Library Module is displaying in Adobe RGB (the preview image). And, of course, the monitor has to have a colour space wider than Adobe RGB, otherwise you wouldn't see the difference. http://www.simongarrett.co.uk/CaptureProPhotoDevelop.JPGhttp://www.simongarrett.co.uk/CaptureProPhotoLibrary.JPG