1) I believe you can add a longer briefing by editing the df2brief.bin file (inside the df2.pff file) using the
BIN Editor
2) Add a .tga image with the same filename as the .bms file.
3) I think this is what the "Briefing Map Center" marker is for in the MED.
4) I don't recall this one. It's probably some combination of .wav files in df2.pff or terrains.pdf.
5) See #1
I haven't done these things in quite a while, thus the vague info. If you happen to read this and need more help, let me know!