Yes, legally and for the computer to function correctly you will need a catalytic converter.
Legally you can only replace a catalytic converter with another catalytic converter.
Legally a defective catalytic converter must be replaced.
Not legallyNot legally
Absolutely not. The fumes released from any engine can be deadly without a catalytic converter and severely harm the environment no matter how efficient the engine is and no car can legally not use a catalytic converter.
Not legally.
Legally, yes
Not legally
Not legally.
Not legally.
You don't not legally anyway. You could put in a straight through exhaust line, but if you're caught you could be severely fined.
Legally, yes. Functionally, no.