If I can clarify the requirement:
1. The Word startup folder is located on a network share and applies to all Office users in the department concerned, via Group Policy
2. The word addins I need to deploy cannot be made available to all users due to licensing.
3. I have been unable to determine whether it is possible to specify a second (or third) startup folder locally.
4. The addins need to load automatically as word starts up. No other user action should be involved.
5. I have created a solution which consists of an autoexec macro which is added to Normal.dotm manually. This does the job perfectly.

1. There is still a Startup locally

2. There seems to be a bit of a conflict here. If these are NOT available to some users, how does this balance to #4 - that they are loaded automatically. Does your autoexec test the username somehow

3 see #1...there is still a local Startup

4. Some addins, all of them...what

5. perhaps post what is in that autoexec.