do you have to recover a mix of nitrogen and a trace refrigerant
No.
To much work with too little time!!!!
Trace gas.
no
No, but you "required" to recover any remaining refrigerant in the system before pressurizing it with the trace of refrigerant and nitrogen.
Weird, but that's how it works.
You can even turn around and use the recovered refrigerant as the trace gas...
No +5