You can clear the codes without a code reader, or waiting. Disconnect BOTH Batt - (negative/ground) cables (or all), and connect one of them to a Batt + cable (leaving BOTH negative battery posts/terminals connected to NOTHING, do not short circuit the batteries), connecting only the CABLES together, not the batteries. Leave for 30 minutes to an hour. Reconnect the cables and the codes/light should be clear. Just disconnecting the batteries can take a day(s) to clear the computer.

I've heard of, but not personally seen that I can recall (I've seen conditions that could have been, but not proved), an "open" exhaust cause EGR issues. Theoretically it is possible, as the expected manifold pressure change may not be sufficient enough to satisfy the computer with less exhaust back pressure. If the Cat is gone, it could be a more likely issue. If you reinstall the original exhaust system and the problem continues, then it isn't the exhaust system.

The additives may not help with EGR issues, as they leave the cylinders as gas and soot, just like the fuel (which is a problem in the first place). You can remove and clean the EGR valve manually, though.

Also, make sure you have sufficient vacuum to operate the system (test the vacuum pump/system).