I'm gonna shoot straight with you, please don't interpret bluntness as negativity towards you. If anything, it is towards the pool store.
Non-chlorine shock does not magically make the baqua disappear. Your inability to hold FC and CC readings indicate that there is still baqua in your pool that it is reacting to. As for the pool store not mentioning changing the filter sand, no surprise there. Right now your best bet is to go back to the start and treat your pool like it is still full of baqua and go through the process I posted above, except since your CYA is 60 then you need to target 24 for a shock level, not 15.
Alternatively, with a pool that size it might be cheaper to just dump the water and start fresh. You will still need to change your filter sand and the residual baqua in your hoses will cause some minor FC demand, but nothing major.
Also, when you say green, do you mean cloudy green or clear green? Actually, this is a pretty important question, could you answer this before you do anything else?