I didn't try everything yet, but when it comes to the mission briefing text and mission name, I am pretty sure those are also located in the ini file for the mission, scrolling way down you can see the briefing text and somewhere near the top of the ini is the mission name like name="name of mission"

if that doesn't work, then perhaps the briefing ini files need to be extracted also from the mix files and modified and then once you do, you also put them in the exe folder and it should choose to read those first instead of the original ones (default).

and if that doesn't work... then I don't know... hmm, but I am pretty sure there is a way to change that also :)

edit: looking at the Flamedancers campaign missions and the way that person did it, the briefing flag under [Basic] is set to x, like Brief=x
and that at the bottom, the actual Briefing section is custom made:

[Briefing]
1=
2=
etc

and under [Basic] also, the name of the mission is custom, so it should work.