Installed a Residential Gas-FVIR Certified Water Heater. Problem is the hot is backing up into the cold water line... HELP

When you say, "the hot is backing up into the cold water line." do you mean it's just backing up into the cold water feed or is it affecting the entire system? Let me know, Tom..

Backing into the cold water feed

This is too complicated

What's so complicated? Thus is simple convection. The heat from the boiler travels back up the copper feed for a foot or so and then stops. This is a normal reaction to heat. Chill out! You're just fine. Cheers, Tom