Gowron actually had some shoddy security in those two episodes. In Redemption 2 a random guy just walks up, draws a knife and fights Gowron for example. Even a relative nobody like Kurn boasted about being able to kill Gowron and I don't think it was idle.
Then he must have studied up on melee because he nearly killed Worf in DS 9.
As for the shoddy security his klingon bodyguards probably cannot stop a challanger from taking part in honorable combat.
If Picard had ruled the challenge valid, then the assassin would have killed Gowron and Toral would be left as the only challenger for leadership, much like it was with Gowron when Worf killed Duras.
That little punk would get challenged and killed immediatly. if we want to retcon it here is an explanation:
If Gowron dies instead of the civil war having 2 camps it would have 3 or four because only Gowron can unite certain houses to fight together.
By the way if Gowron would had said here and now Toral would it had been owrite or do they have to do that in a sacred chamber at x pm?