How does Pyke Aggression and Boldness and Courage combo?

They almost don't. The B&C second part of the ability get's overruled by the Pyke Aggression limitation on "Melee Attack using it's lowest Attack Die value".

It depends

It depends, here is the scheme based on the rank of the attacking unit which has B&C and uses Pyke Aggression :

1 rank - lowest dice value 2 rank - lowest dice value 3 rank - lowest dice value +1 additional dice