Regarding your wonderings about Kevin's plans.
Here's Mhoram about Kevin:
And Atiaran said:"The age of the Old Lords, before Lord Foul broke into open war with Kevin son of Loric, was among the bravest and gladdest and strongest of all the times of the Land. Kevin's Lore was mighty with Earthpower, and pure with Landservice. Health and gaiety flowered in the Land, and the bright Earth jewel of Andelain bedizened the Land's heart with precious woods and stones. That was a time-
"Yet it came to an end. Despair darkened Kevin, and in the Ritual of Desecration he destroyed that which he loved, intending to destroy the Despiser as well. But before the end, he was touched with prophecy or foresight, and found means to save much of power and beauty. He warned the Giants and the Ranyhyn, so that they might flee. He ordered the Bloodguard into safety. And he left his Lore for later ages - hid it in Seven Wards so that it would not fall into wrong or unready hands.
It seems to me that Mhoram's story says that Kevin didn't have much time to throw the Wards together, while Atiaran's suggests it was a well-thought-out plan."For a thousand years, Kevin stood at the head of the Council, and he extended the Earthfriendship of the Lords beyond anything known before in the Land, and he was greatly honored.
"In his early years, he was wise as well as mighty and knowledgeable. When he saw the first hints that the ancient shadow was alive, he looked far into the chances of the future, and what he was gave him cause to fear. Therefore he gathered all his Lore into Seven Wards -
Seven Wards of ancient Lore
For Land's protection, wall and door-
and hid them, so that his knowledge would not pass from the Land even if he and the Old Lords fell."
As far as the Ritual goes, I have become convinced that, though he doubtless despaired, it was a carefully considered plan. In fact, the only thing he could do! Foul was going to win. No question about it. He was too well entrenched by the time he declared open war on Kevin. (I picture him as Darth Sidious, turning Lords against Kevin, or at least making them follow himself more than Kevin.) If Foul won, the Land would suffer. Probably forever. Kevin may not have even expected the Ritual to destroy Foul, though he may have hoped so. But he knew that it would at least slow Foul down. And as Foul recovered, the Land and people would also recover, and they just MIGHT have a better answer to Foul. Either find a way to spot him before he got so far; actually defeat him; or figure out a good way to use the Power of Command.
It all hinges on the fact that NOTHING else Kevin could have done would have ended in any way other than Foul winning, and ruling the Land.
