Simple question, the answer to Exercise 5.6 Qeustion 3 in the Performance book states that the maximum cargo which may be loaded on to the aircraft is 555 Kg.

The maximum permissible load for baggage and cargo compartments is 320 Kg (55 Kg left, right and forward compartments, and 155 Kg in the rear compartment), so given that the flowchart allowed 555 Kg, would 320 Kg not actually be the correct answer?

I wrecked my brain trying to figure out if I was missing something, in the process I realised that seat's can be removed and 82 Kg of cargo can stand in their place, but this weight would surely not be classed as cargo for the sake of loading if it is in the passenger compartment, right?

You will notice that in that question, there are no passengers on board. Any cargo placed on passenger seats is still called cargo. The fact that it is in the passenger compartment does not alter that. You certainly couldn't call it passengers. It would be listed in the cargo manifest as cargo.

Ok, so if I have this right, providing the question has no passengers, meaning 5 seats can be removed (Allowing an extra 5 X 82), and if no other weight restrictions were to limit you, you could actually load a total of 730 Kg of cargo (Disregarding COG), in theory?

yep