I feel comfortable answering that here, but maybe if I could find the original 80-page plot outline we started from, that might be a cool feature for the Toolbox. Unfortunately, I just did a search in three places and can't find it. Maybe Jamie still has a copy, or it's archived in GMail. I do have the original scripts I sent to Tim Buckley when we were considering developing this story back in 2003, when it was called "Kenny Bogansky, Interdimensional Warrior." Those, I think I will probably load to the Toolbox.
So anyway... Originally, the book was plotted more or less to the ending you read. Parson blows up the volcano by having Sizemore set a magical dig spell going straight down(the tri-mancer thing was worked out a little later), Wanda gets the pliers, creates "super uncroaked" of the dead, Sizemore finds all the gems in the aftermath of the volcano, Stanley returns to an all-powerful side, and Parson is left wondering just what he has done.
It was the middle that got more convoluted and complex, starting at about the time Jillian throws off the spell. Vinny was going to die, the archons were not meant to play much of a role, and definitely Charlie was not originally intended to be so pivotal. Bogroll was going to live, and we had a great gag set up, in which someone gives Parson a personal toilet that's made out of Jillian's abjuration hat. Then we see Mung or one of the other henchmen giving Bogroll the conjuration hat to wear.
But I'm glad Bogroll found a a bigger destiny and apotheosis.
So to answer the question, Book 1 was meant to end this way, and meant to continue to future books. But if we had decided to ditch the project after one book, the ending would have been something more final. I would have considered either killing Parson (original conception of his character was deeply depressive and carrying a death wish), or sending him home. Fortunately, it became clear that the story would/should go on, so we stuck with the ending we were setting up all along.