Your FC constantly dropping means you are DEFINITELY fighting something.
Shocking DOES push your pH to read high but I would still add some MA or dry acid a bit at a time till pH reads under 8, in the 7's. When FC is high, I'd just like to see pH under 8. When/if FC drops again, you should see your TRUE pH and you'll want it in the 7.2-7.5 range

I have no idea how you can measure calcium hardness and get a number like 17.1ppm. I'd be hard pressed to tell if it was 10 or 20ppm. Also, if, at some point you were using cal-hyp, frequently packaged as "shock", that adds calcium. Even my local tap water tests around 60ppm.

Still, you may be bang on accurate and outside my experience.

Carl