Levalt
From what you wrote, this is my understanding.
For each of your projects, you set up a special save location for the project file (project.prel) and the source media that go into that particular project.
If you have 12 projects, for each, you have a special save location (the project.file source media combo.)
Each of those 12 projects needs to be exported to a file saved to the computer hard drive. One suggestion might be is to save the export from each into a saved location name Grand Project Source Media.
It is these 12 exports that are going to be imported into the Grand Final product which will be outputted as wanted. At this point, those exports are your source media that needs to stay in contact with the grand projects. The source media that went into the individual 12 projects are out of the equation at this point.
But, back to the point that I was trying to get at. Each of those 12 projects that you start with has to have project settings (aka project preset). And, those project settings should match the properties of whatever you are putting on the Timeline of the each project. The project settings direct the program to set the right space in the Edit area monitor for editing so that you do not have black borders, pixel aspect ratio problems, and the like. So, I want to make sure that you have the right project settings for each project.
Then you need to export the TImeline content from each of those projects. Ideally the export settings should be consistent with the source media properties and the project settings. The ideal is to have this to be the case for all 12 projects. Cannot advise you about these settings without details...
What is the project preset for the 12 starting project...NTSC AVCHD Full HD1080i30 or something else?
If that is your project preset, what are your source media 1080i30, 1080p30, or something else?
I am suspecting that each individual project will be exported to a HD export 1920 x 1080 which might be a Publish+Share/Computer/AVCHD with Presets set to maybe a .m2t file, maybe .mp4 file. It is all going to depend on a coordinated plan.
If you still have questions and need clarification about the above, do not hesitate to ask. I will explain another way if necessary.
Thank you.
ATR