Premiere Elements 12 on Windows 8 (assumed 64 bit). Have you updated 12 to 12.1 Update yet? If not, please do so using an opened project's Help Menu/Update.
Please check out the following which gives a detailed workflow for how to gather, assemble, organize, and manage video grabs from a large collection of video files going into one Premiere Elements project.
What is the format of the large files that you need to trim....VTS_01_1.VOB and the rest of that series or other?
Where are these video files now....on the DVD-VIDEO on DVD or video files from the DVD already copies/ripped from the DVD and
now saved to your computer hard drive?
I ripped each VHS tape to one Mp4 file, stored on my computer. In Premiere elements, I took each 4-hour-long file and cut into clips and saved as .prel file. I want to take those clips and rearrange onto different .prel files for individual dvds.
Just in case note, you cannot import one Premiere Elements project file (.prel) into another.
So if you already have several project files, each with its own group of files and want to combine them into one grand project for a DVD (DVD-VIDEO or AVCHD), then you are going to have to export the contents of each project prel and import the exports into one grand project.
The export from a given project.prel can be for the entire Timeline or for selected portions of it. But the only way out of the project is via export. In this situation the export settings for the individual project files should be consistent with the settings for the grand project which will generate the end product.
To export a segment of a Timeline content within one of these project files, two essentials
a. the gray tabs of the Work Area Bar must be set to span the segment for export
b. the option "Share Work Area Bar Only" in the export settings must have a check mark next to it.
When your Premiere Elements is running on a Windows computer, there is an opportunity for a copy/paste insert between projects.
Please review the details of that if that sounds of interest.
Please let me know if I am targeting your questions. If not, please give more details, and I will modify my reply accordingly.