I can see several potential issues.
- First issue is the access to other projects. I do not know how access is controlled within P6 but this I would try to solve with the software administrator so you can work it in a simple way.
- Second issue is with having all jobs consolidated on a single Portfolio, something you shall do, in such portfolio activities ID would be changed by adding some additional code characters.
In Spider I would try the following.
- Consolidate all jobs into a single Portfolio, this will solve the issue on activity coding.
- Create my formula to make the desired Global Change on all jobs.
- Distribute back all jobs at a single click.
- Use Reference Books to import data.
- Delete the portfolio if no longer needed, it will delete the Portfolio tables but not the individual projects.
Still I would need access to all jobs in order to be able to tamper with the data tables otherwise access control would be useless. I would not count on bypassing access security so easily.
In Spider the job files are transferred with the access control, not sure you can bypass security with Primavera xer files. In Spider if you want the recipient to have full access and you defined some access control you will have to release the restrictions as it will retain access control unless you specifically want to change it.
The following figure will show you how activity IDs are modified by the software so it does not get confused. For simplicity I used two versions of the same job so some [so many some becomes all] activities IDs are duplicated. Also you will see how each job has its own code.
I hope this can gives you or someone else some clues on how to best handle your task. I believe you shall be able to do what you are looking for if given adequate rights to access all jobs, for your global change as well for your SDK data imports.
For data imports we use what we call Reference Books, but we are looking to solve it with what you have so I am not including the details of how I would to handle the imports, I suggest using SDK as available in P6, it shall do the simple job of adding the activities.

Good luck.