Nice work, as soon as I have time I am going to check this out in detail. Thank you so much for sharing.
I have been working on a pretty easy model by using the existing vb javascript files "vbulletin_quick_reply.js", "vbulletin_quick_edit.js", ... anyway, you already know the problem with that method -- so many different editor modes to modify. After staring at the code for awhile, I too was thinking that I should just position the data via the DOM and be done with it. Since essentially that is all that is being done in the vb JavaScript files.
Sorry, I missed your first (previous) post somehow. I must have been traveling for work or I would have answered, but it looks like you were a bit ahead of my work. I just finished "quick_reply" and "quick_editor" two days ago and still needed to verify caret positioning among the browsers.
Good work! -- I am impressed.