1- Activated carbon can remove most of medicines like CP, General Cure ... from rocks, substrate, except ionic & chelated copper?
Yes, but only if the medication being stored by the rock has been released back into the water column. At least with copper its a back & forth situation - the rock absorbs copper, releases copper, rinse & repeat. A poly filter or Cuprisorb is best at removing copper.
2- Coppersafe, Copper Power are chelated copper. Coppersafe instruction is 1.5 - 2ppm and Copper Power is 2.5ppm. Somebody told the instruction of Copper Power was wrong. I've got a bottle of Copper Power on hand, should I keep 1.75ppm when treating?
1.5 - 2ppm is therapeutic range for chelated copper. Copper Power recommends overdosing out of an abundance of caution because most fish will handle 2.5 just fine. However, 1.75 is a good "middle-of-the-road" to aim for.
3- I've got a quarantine tank with live rocks, I think it's not good if treating fish in this tank with copper or CP. It's will be better if the fish is treated in the tank without rocks and substrate. Am I right?
Yes, however a sand substrate won't absorb meds as much as rock.
4- I saw your instructions from humble.fish. That the treating period can be shortened by transferring fish to quarantine tank after 2 weeks of treament. Do you think 8 days treating with copper or CP + tank transfer method (transferring every 48 hour x 4) can eradicate ich and velvet?
Technically yes, but 8 days is cutting it a little too close IMHO. 14 days allows for more leeway and margin for error.