Somebody oughta PM Delc on this one. He'd be all over it.
My understanding is the Hiei's armor was upgraded to BB standards between the wars.
The three BCs that blew up at Jutland were all slugging it out with German BCs when they went bang. Not BBs. There were BBs in around but they had not yet gotten into the that fight. Well, not affecting the British BCs anyway. German BCs took down the British BCs at Jutland, not BBs. The British BCs blew because of poor ammo management and the volitility of their powerder. The Germans had had a similar problem but a fight after a BC raid on the British Coast (where the Seyditz lost a turret and almost the ship to the same problem) taught them Ammo Management ("Ordanance instructions are written in blood"). Their powder was less volitale from the start.
The Hood had been upgraded in a few spots, but not continuously. Not sure of the details, but the round that put her under seems to have been a freak shot. In the parliance of sport, the round had eyes. It went on a somewhat ecclectic route to the the secondary battery magazines. They detonated and took the mains up. Given the route it took (past or around armor plate), it entirely possible that a round from the Prince Eugen could've done it. But the Hood had just been on the receiving end of a Bismark Salvo when she blew so the odds are very good it was the Bismark that fired the shot.
Another point, the wreckage of the Hood is in such a manner that their is credible evidence that her forward magazines blew up also. The final shot, where the forward turret fired as the ship sank, could've been venting from that event instead of an actual salvo.