Re , just split the window as usual. When you activate the split, Scrivener defaults to a second pane containing the same document you’re currently looking at, so you’re set.
To do it with a different document when the view is already split, place the same document in each pane in the usual manner (i.e. by dragging to the pane title bar, or selecting the document in the Binder when the pane has focus).
Re : Have you thought of using Grab for this? I’d suggest:
Open images in Preview.
Switch to Scrivener and put the insertion point in a document.
Select Services>Grab>Selection then select the part of the image you want to use. Scrivener will paste it directly into your document at the insertion point. (If you don’t follow all of point 2 then the Grab sub-menu will be greyed out.)
I use this method to get screen shots for a Windows-only program that I’m documenting, with the software running in Parallels on one screen and Scrivener on the other. Another benefit of working this way is that you can have screenshots and text in the same documents.
A) From Selection to Scrivener: Scrivener has two Services items that are available in any Services compatible application. One of the items is: Services>Scrivener>Create New Clipping from Selection. So, this looks like it might be what you are looking for. (If the selection in question embeds an image, the image will get imported too.) Assigning a keystroke to this Services menu item greatly increases its utility.
B) From Clipboard to Scrivener: I don’t know of a single keystroke that would get the clipboard contents into a new document. I guess if I wanted such a function, I would use my key macro program to assign a keystroke to the following three key sequence: command-n (create new document) followed by control-option-command-e (shift focus to editor pane) followed by command-v (paste). But, again, the Services item described in (1) just makes it better to bypass the clipboard and go right from a selection into Scriv.
I think the others have answered your other two questions (2 and 3 are 100% possible and current features), but as for 1), no, there are no plans to implement 2-way linking.
All the best,