How can I prevent dashed lines from printing in PDF with tiny hooks?

re: Horizontal; one spans two cells, the others just one; line end style is "no arrow."

You might try the rounded end style.

re: It doesn't show up on a printed hard copy, just in the electronic PDF version.

Just in Adobe Reader, or in all PDF readers (try the one in your phone)? This may actually be a client (reader/viewer) problem, and not something FM is doing, much like the stray lines when raster has odd pixel count in a line.

Worst case: if the lines required are all the same length, or a small number of variants, you could create them externally in a vector editor, and import them on top of the table at SVG or EPS.