The "shock plus is dichlor, which will add chlorine, slightly lower ph, and begin to raise cya, so this is an ideal time to use it. The pH down is dry acid, and you could dose it the way you describe, except I would only add 4 oz or so, since the dichlor may already drop the ph. You don't want it to get below 7.0, because that can damage your liner. I would wait 2-3 hours after dosing to check results. Remember that it's better to sneak up on your target nimber than to overshoot it. Your filter is probably a cartridge and cant be backwashed so you can spray it off with a hose.

Janet