Put both files into the SUN folder. battle.ini and BattleFS.in.i
Change out the number within the line Scenario=GDI12A.MAP
into a lower one then start a new campaign and you will play the mission you selected with that number.
The campaign can not be proceeded with this method when the mission is won it will continue with mission 2 but you can exit and change out the number which takes some seconds.
The correct briefing is shown when you look them up. Begin & End Videos wont play.
Be aware some missions are called 10B or so since there are multiple choices for the campaigns. I am looking for a campaign mission list with the numbers and
will also list which numbers are for the Base vs Base missions. I will post this later once I found it. I have set the numbers so the campaign will start with the final GDI mission and the 9th Nod Mission where you Establish a Nod Presence.
AFAIK it appears to be the best method to check if missions work with mods. I didn't check but there is a chance that the ini files within the mix files have to be deleted in order for this to work.
This also works for custom campaigns, place the custom campaign map file into the SUN folder and rename it to GDI12A.MAP or whatever mission name is written within the ini. However the briefing will be wrong.
I invite a mod admin to create a spot in the media hut for inis or at some other place. If you happen to find a more fitting spot for this in the forum then move it there

The MODDB news http://www.moddb.com/mods/orange-touch-up/news/mission-selection-tools-ts-fs
The mod OTU for which I test it (shameless advertising indeed) http://www.moddb.com/mods/orange-touch-up