Nothing like a deadline, eh?
Since your handcoding everything, just theme zenphoto to match your site. All the theme files you'll need (including the css) are located in the default-dark theme directory, and you can just change the headers and footers to the ones you're using for the rest of the site. Should be relatively straightforward. Just look at the code in the default-dark dir for an idea of how to lay out the css and html. Might take a bit of poking around in the code, but it shouldn't take long to adapt your site html/css to zenphoto's theming. Just remember to edit index.php, album.php, and image.php with your header/footer/css information, and make sure to take note of the zenphoto "loops" and other php declarations as you go through it.
That way, when someone clicks on "gallery", you direct them to the /zenphoto folder and the zenphoto theme will take it from there. You're not having to mess with any sort of redirection tricks with code then. I would suggest getting it working without mod_rewrite first, and then you can mess with pretty URLs later.
Let me know what you think from here, and we can proceed with any theme questions.