Hi there, with any U1000 and U1001 code you will need to perform CAN diagnostics.
A nissan dealer can easily perform CAN diagnostics to see the control unit not communicating. Since you actually have driveability issues and not just a U1000 code then you may have an actual failed control unit instead of a grounding issue.
I assume the dealer was referring to tracing the CAN lines and inspecting for damage which is highly unlikely.
You need to clean and inspect the ECM grounds at the dash beam and also by the battery. If cleaning and retightening grounds does not resolve the code then CAN diagnostics will have to be performed at the dealer.
Sometimes if you have a regular scanner that cannot run CAN diagnostics you can unplug the control units one at a time.