Hello, I hope the following assists you.
A fault with the EGR valve will give the symptoms you describe, any exhaust gases entering the intake system when they shouldn't will starve the engine of fresh air so causing an over fuel condition hence the black smoke.
The EGR valves do give concerns on the Cmax most tend to be the valve has stuck.
The injectors on this engine don't give too many internal faults but externally it tends to be a case of the injector seals leaking which causes the combustion gases to blow past the injector, this can usually be seen by black molten plastic around the injector and you may hear the noise of the gas blowing past although to affect the engine running this needs to get very bad to the extent where the affected cylinder is losing compression.
You first need to pinpoint the cause of the EGR code and eliminate this before even considering a leak off test on the injectors, to be honest a leak off check doesn't usually show too much on this engine unless there is a serious injector fault.
The EGR circuit low is the PCM measuring the voltage to check the EGR valve position, as it is low this suggests either the valve is in an incorrect position or there is a wiring fault to the EGR so check the connector and the terminals in it for any sign of water damage and security.
I hope this helps, let me know if I can assist further and I'll reply as soon as I'm able to.