Botched short name change attempt has left me with no permissions! Help!

Question

Early on, my MacBook Pro's hard drive ground to a halt and I had to fork out to get it restored. When it came back, my administrative account was no longer named "Will", but "macbookpro".
I changed the username simply, but was always bothered by the home folder being called "macbookpro". After buying a WD MyBook and backing up my "macbookpro" user folder with the WD Anywhere application, I (stupidly) decided to follow an outdated Apple guide ( http://support.apple.com/kb/HT1428 ) on how to change the short name via Terminal and root - something I now realise is no longer necessary in Leopard.
When adding the new account to replace "macbookpro" with, I couldn't, as the existing account had the username I wanted already, just not the short name. I changed what I thought was the username for what I thought was a temporary amount of time from "Will" to "William", so they had different names and I could proceed.
I followed the rest of the guide up to the point where I log on as new user. Forgetting I wasn't finished, I fired up Adium - which acted like it had never seen me before (something I don't really mind), and to this moment it has refused to sign in to MSN, but does AIM perfectly.
After being puzzled by Adium I continued the process, finding not "the keychain, which should still have the short name of the original user", but "login.keychain" - I left it.
Giving up and continuing to use my computer, I attempted to update Camino by downloading the latest version whilst simultaneously resetting all of my Application preferences to how they were before.
On dragging Camino to Applications, I was told I did not have sufficient privileges to do anything but view it. Everything except what is in my new home folder is read-only.
Sharing and Permissions shows me that macbookpro is the only account with jurisdiction over Applications - Read & Write, whereas groups "admin" and "everyone" are Read only.
For most other files, "system" and "admin" have Read & Write permissions, except in the System folder where only system has, whilst "wheel" and "everyone" can only read.
In my panic, I renamed "William" to "macbookpro" again, meaning that when I log onto my (now completely reset and devoid of preferences) original macbookpro account, I have my permissions mostly back.
You'd think I would then simply revert to using macbookpro and changing all my preferences, using the Leopard short name change - but "macbookpro" doesn't seem to be able to open ANY applications. At all. For no good reason.
Short of going through every single file on the hard drive manually and changing the permissions to allow "Will" to read and write, I can't solve this!
This puts a massive hindrance on my use of this computer, and I don't want to edit permissions on each file as I need it and then discover the only solution is to completely restart!
Please, somebody help!

Answer 1

If anybody ever has a similar problem, here's how I solved it as best as I could:
http://www.macchampion.com/arbysoft/Welcome.html BatChmod is a tool for batch editing file permissions.
It fixed my Applications and Library, and I can now read and write to both folders.
I dragged solely the contents of Application Support from my backup over the existing Application Support folder within my user's library, and that restored a few, but not all, of my Application Preferences.