I'm a very happy owner of the Galaxy Tab Note 10.1 (2014), and use it extensively for storyboarding - so I _need_ that S-Pen ! I've almost lost the original one a couple of time, which led me to seek for a spare/potential replacement for quite a while.
After reading this very informative (and lengthy) thread I decided to check what Amazon had in store for me - and I took the plunge on a Note 4 replacement pen . I expected to lack pressure sensitivity (something I could live with if I had to choose between this and no pen at all), and a large-ish offset (something I definetly can't work with). But for 10$CAD, I thought - hell why not.
Against all odds, this seems to be the real thing. Most likely a Chinese copy, but good enough to have pressure sensitivity, and no offset (at least no more or less than the original S-pen that came with the tablet). Tip is also similar (sort of rubbery plastic, not 100% hard plastic). Only difference drawing-wise is that pressure sensitivity is bit higher (less pressure required to get thicker line), I can live with this slight variation. I popped the button, and there's not potentiometer under it, just some surface mounted elements, nothing moveable. The lines appear under the tip (or catch up to its position after a quick line) and there is not more or less offset if I angle the pen to the screen surface.
However, since it's designed for the Note 4 and Edge, it does not fit perfectly in the S-pen slot of the tablet : the "new" S-pen is slightly shorter (~1cm shorter). It will fit in the slot, but not "snug". But I'll take that any day rather than having a tablet with no digitizer at all. Besides, I am thinking of getting a second replacement and turn it into a full-sized, weighted pen.
I also tested several S-Pens in stores where Tab A's and Notes were on demo with their pens. Most, if not all, worked fine with the SM-P600, with the same feeling, pressure sensitivity, and no offset. That would be another avenue for those looking for a replacement. I hope this helps anyone looking for a replacement for their S-Pen, I know I looked long and hard everywhere for it!
 : Since it's my first post, I cannot post an outside URL. Look for "Samsung Original Stylus Touch S Pen for Samsung Galaxy Note 4 SM-N910 & Galaxy Note Edge SM-N915" on Amazon.