ok, so i finally came up with a solution that works. i can now change the number of evenly spaced vertical partitions such that individual shelves (or sets of shelves) span the exact distance between them, making the lines that define ends of the shelves visible as they would be in reality:
http://imageshack.us/a/img17/6784/jxkc.jpg
the explanation of how i got here and the hurdles i had to overcome would be quite verbose, so i'm just going to upload the component for you to download and play with. if anyone has questions about how or why i might have done something a certain way, feel free to ask. also, if anyone has suggestions on how to make this cabinet component less complex geometry/rendering-wise, i'm all ears. i know there's probably more geometry present than is necessary in order to make this cabinet work as i intend, but i'm a beginner, so that's to be expected...i don't know all of the ins and outs of SketchUp yet.
thanks,
Eric
PS - how difficult would it be to add a 2nd predefined "Copies" attribute (not my own custom attribute) to the component attributes dialog box?...or is it even possible? this would make my model simpler with respect to the amount of geometry present with the component in its most basic form (0 shelves, 0 vertical partitions), if not in the amount of overall geometry rendered when many shelves and/or vertical partitions are present.
W2D_shelf_width_solved.skp