The convection oven is 1-3 phase 11KW. The service is 120/240V. How to rewire the oven to single phase hook-up. Does it require a disconnect or can be cord and plug connected? What about grounding and neutral?
Do you mean that this is a Single-Phase or Three-Phase compatible appliance? If this is the case, you will have to refer to manufacturers instructions and specifications on how to make the internal conversion.
You other questions regarding proper grounding should also be covered by the manufacturer.
I have never seen or ever heard of this type of arrangement, so this is all I can suggest to help. Good Luck!