Sorry for your trouble. First question, did you add enough sequesterant for your pool size? It is always better to add more than enough versus not enough. Second, you may have some algae forming - with a cya of 50 you should not have chlorine under 3 - you need to keep it around 5. The ascorbic acid (vitamin c) will bring the chlorine down to 0, and will continue to bring chlorine down as long as there is ascorbic acid in the water - that's why you bring your chlorine down to 0 efore the stain treatment, because the ascorbic acid gets eaten by chlorine. The bringing the ph up to 7.8 is for trying to get the metals to stain the filter, he wants you to put a trichlor puck in the skimmer basket - this way when the high ph water hits the chorine puck, it makes the metal precipitate out of the water and land on the filter. I don't try to do this myself - I just add the sequesterant and keep my ph low for the first 2 weeks after a stain treatment, so that as I bring up my chlorine levels they are less likely to precipitate the metals. I also keep a chlorine puck in the skimmer. You don't want to put a chlorine puck in the skimmer if you have a cartridge filter - only a sand or DE.

This is what I would do - make sure you have enough sequesterant in the water. Bring your ph down to 7.0 to 7.2. Put a trichlor puck in the skimmer basket and keep your pump running 24/7 (not if you have a cartridge). Add polyquat. Start to bring your chlorine up, and watch for stains (keeping the ph low will help not to stain) If you see stain forming, put more sequesterant in the water. Keep adding more and more chlorine - be consistant - don't let it fall lower than you had it , if you have it at 5 do not let it drop, add more to get it to 7, don't let it drop, keep doing this till you get up to shock levels.
You may have algae, so you will have to get the chlorine up to shock levels, which for you will be 15ppms. It will be hard at first, because the chlorine will get eaten up by the ascorbic acid and the algae. Keep the filter running 24/7, backwash everyday, and if you have a sand filter add a cup of DE to the skimmer, or you can add a skimmer sock (or a cut leg from pantyhose). This will help filter out small particles. Keep us informed, and ask any questions you may have.