PaulDoesWP on "Subfolder installation AND masked domain forwarding"

Hello.
I have a WordPress site: http://iambiscuit.com/ which is hosted at http://paulroper.com/iambiscuit/

My WordPress install is in a subfolder ( /iambiscuit ) in the public_html folder of paulroper.com.

I have set up masked domain forwarding (with Godaddy.com) so iambiscuit.com forwards to http://paulroper.com/iambiscuit/. I have been having all sorts of problems with WordPress just refusing to get to grips with this. In Settings > General, I have: the WordPress Address (URL) and Site Address (URL) set to http://paulroper.com/iambiscuit. My problem is that any generated links are in the format http://paulroper.com/iambiscuit, and not just iambiscuit.com. For example, today I added the official Twitter plugin, so someone can tweet one of my posts. But when they click on the tweet button, the generated link is in the paulroper.com/iambiscuit.... format, not just iambiscuit.com (I have since disabled that plugin).

I have tried tweaking the Settings > General WordPress Address and Site Address to make either or both of them just http://iambiscuit.com, but as soon as I change either one from the full http://paulroper.com/iambiscuit/, the site dies completely, and the only way I can resurrect it is to add

define('WP_HOME','http://paulroper.com/iambiscuit');
define('WP_SITEURL','http://paulroper.com/iambiscuit');

to my wp-config.php. From then on, it is impossible for me to ever edit the Settings > General > WordPress Address (URL) and Site Address (URL) in the Dashboard, even if I delete those WP_HOME... lines from wp-config.php. (If I delete those two lines from wp-config.php, the site is completely inaccessible and dead).

I also found these in wp-config.php:

define('SUBDOMAIN_INSTALL', false);
$base = '/iambiscuit/';
define('DOMAIN_CURRENT_SITE', 'paulroper.com');
define('PATH_CURRENT_SITE', '/iambiscuit/');

...and wondered why SUBDOMAIN_INSTALL is false. I also wondered if tweaking any of those other values might help, but haven't tried yet, because I am kinda tired of my site falling over at the slightest change!

So, what I'm looking to do is remove any reference to paulroper.com, and make all my links start with iambiscuit.com. If I look at my permalinks settings, all the options start with http://paulroper.com/iambiscuit/...

Any suggestions would be greatly appreciated!

- Paul