I'm starting to think that all the problems we're experiencing may be related to file ownership. I'm not 100% sure if this is something that is modified properly (if at all) when switching to a new Webhost so I want to test the theory.
In order to do this I'll need to create a test site ensuring both aesthetic and functionality are the same as current on a new database.
Then I'll re-route it to the new database and retest.
This of course means there will be some downtime to test on the new database so I'll have to take this forum offline a few times to do it. I'll be sure to give you all plenty of notice prior to testing and would appreciate if a few of you could help me stress test.
If it works I'll wipe this install replacing it with a fresh XenForo install.