Does using activated carbon reduce or eliminate the need for water changes?
No, water changes still remain an essential part of maintenance.
Are all carbons designed for aquarium use ?
No, usually quite the opposite. Most carbons are destined to be used in the chemical industry and air purifiers. Many are treated with phosphoric acid, which carbons for phosphates by introducing a few fresh 10 minutes, test the water with a phosphate test kit. Those carbons revealing no phosphates are to be recommended.

Can a good Quality activated carbon help restore flagging sessile invertebrates?
Yes in many cases, especially with a 33% high quality water change.
Does the use of carbon eliminate the need for a protein skimmer?
No. Skimmers and carbon remove different pollutants and one can only complement the other.