I find it difficult to figure out when to believe what various characters say. When
samadhi touched him, Mhoram learned that they had been wrong about the Colossus' purpose all along. And how did anyone know that Kevin died hearing Foul's laughter? Foul told Covenant in the beginning of LFB, but it seemed to be common knowledge throughout the Land. Kevin didn't tell anyone that he was planning the RoD, because he knew they'd try to stop him. So then how did anyone know that he was mad with grief or in despair?
Anyway, about the debate, I don't think that any defensive position could prevent an immortal, indestructable evil, like Foul and the ravers, forever. That kind of evil force will never stop trying. It will always find a way through your armor. I think the only way to be safe from it is to damage it as much as possible.
I think that's why Mhoram didn't just set up extremely strong barriers, or anything else. He took the raver down.
And I think Kevin learned, after trying SO much else, that he had to hit Foul with the most powerful thing possible. (2nd most powerful, since he didn't use the 7th Ward. But he didn't want to damn the Land with unforseeable consequences, only the horrible ones that he knew it could survive and recover from.) So the RoD was he only answer.
Actually, I
can see it just as easy your way. I can make sense of it both ways. But I'd rather give Kevin the benefit of the doubt, because I don't like the idea of someone of his power and knowledge becoming such an idiot. His knowledge of Earthpower - its nature, uses, possibilities -
far surpassed Mhoram's, and he was prophetic also, and I don't like to think that, after trying so many different things, he was unable to figure out a solution that was so unbearably obvious -
You can use the power of the Ritual for things other than the Ritual. Too easy. If it was suggested to him, I think he would have given a sarcastic, "NOOOOOOOOOOO! Really???" It was a revelation to Mhoram, because they were all so conditioned about the Ritual and despair. But I like to think that Kevin understood that, and
tried those other uses, but they were unable to stop Foul. Block him briefly, I'm sure. To "what if", Kevin may even have been able to put up a barrier around Revelstone that Foul would NEVER have been able to breach. But then maybe the rest of the Land would be at Foul's mercy, while Kevin watched the horror from safety. Who knows?