Any movement that will lead to stretch on your scar will lead to increase in growth of the scar like pectoral exercises. You need to get it evaluated and get treatment for the same by either injections or pressure therapy. However keloid is not a contradiction to gym. U can continue exercises after telling your instructor that you need to avoid pectoral exercises.

Keloids could increase with frequent friction or irritation. To reduce the size of the keloids you could get sessions of inter lesional steroid injections at the site each session at 3 weeks interval

It depends on the maturity of the scar. If it is a long standing, non erythematous scar without itching which hasn't been growing in size it will not be affected. Do share your pics and consult online for further discussion.

No... possibility of new keloid remain.. for detailed information do direct online consultation

No, it doesn't,