It sounds to me like you need a life line. I would probably put it on the last "normal" row before the decrease (just insert some smooth, contrasting colored yarn through all the stitches on your needle for that row). To be safe, I would recommend writing down which row has the life line (just in case you need to add more "normal" rows for some reason).
Then work your decreases until you're happy with them. If you find that your attempts aren't working out the way you want, simply rip back to your life line and start over.
Your cables and knitting are beautiful, and changing patterns is just as normal as apple pie (at least, it's that normal around here).