So the belt was damaged
Thus far this is the first time I've heard about the camshaft seizing and there is only one reason why that would happen and it is poor oil supply. If the whole engine was locked solid I would say check the crank as well - hydrau-locked combustion chambers will do the same and can result in bent con rods.
If you are getting good oil pressure then there has to be a blockage - the most likely cause I can think of is an incorrectly fitted hard gasket but I think you would have known about that already. Either way you are looking at a head rebuild (I know you can take the cam carrier off without removing the head but there is no oil supply to the carrier box so get the head lifted as a precaution).
With the head off check the pistons are moving correctly by spinning the crank by hand. Make sure the oil paths from the pump are clear (the side that has the oil filter on it). You also need to check that the valves are ok, the same for the seats.