Different Themes for Different Pages

I have two goals here, and any help pointing in the right direction would be helpful. I’ve looked all over Codex and I’m not finding what I need.

1. I’d like to be able to create a page that uses a different theme from what the main blog is using. I can think of two ways to do this… one is to create a custom PHP page outside of the wordpress system and handle loading the wordpress files myself, the other is to create a page template that has some PHP magic which tells WP: Don’t use the normal theme, use this one instead.

2. I’d like to write a plugin that would allow a registered user to view the site using any installed theme. They would get a “presentation” menu for themselves, but when they changed the theme the change only showed up for them.

I feel as though there is probably some variable or function in the wordpress system which points WP to the correct theme, but I’ve had no luck finding it… I feel like I could accomplish both of the goals above if I could get a line of code like this:

<?php $wordpress->useTemplate(‘CustomTemplate’) ?>

I’m just having trouble with the magic words.

Any help would be greatly appreciated.

-Pete

PS: Is there a better resource than Codex for WP variable and function reference?