Firstly back up your whole Wordpress site using ftp then you can try using this plugin to change the path to uploaded/stored media
Yes according to the author you can change the path to whatever you want. But let me know how it goes
Also if you have pasted the script for UPLOAD in the wp-config. Read the post by NICO in this blog about half way down
Does the external server have 755 permission?
Yes that's correct but you need to give read/write permission to the Wordpress folders.
Here is some good information
The one you're trying to upload to
Please read this Wordpress blog