Can anyone confirm for me that the User Profile to SharePoint Full Synchronization timer job must run in order to remove ophan ad users from a site collection? From my understanding this will remove the record from the profile database and then remove the user from the site collection's userinfo table.
A full synchronization is not needed to remove the entries from the User Profile database only a incremental followed by My Site Cleanup. See Account Deletion and SharePoint 2010 User Profile Synchronization
But this will not remove users from the Site collections userinfo table, there is no automatic deletion happening here. If you want to do the clean up you can use Clean-up User Information List
By and large, most companies do not want orphaned AD users removed at the site collection level. Consider the following scenario:
If you remove Person A from the site collection after they're removed from AD, you lose all reference to Person A in the Author and Editor fields. 5 months down the line, if your clients want to know whether Bob from accounting who got canned wrote the 3rd draft of this proposal or if it was Angela from sales who moved on to a better position elsewhere, you might just be out of luck.
My general recommendation is to leave users in at the site collection level unless AD is reusing login names, and even then only remove the users from site collections the new user is utilizing.