I have finished developing mission 13 of the mod, with some edits by McBenn, and it is now ready for download. The intro of mission 12 has been prepared to take you through to mission 13, as though it was finished.
The query choice at the end of Ar11 will take you to Arabian branch, but there is a hack to switch between these paths without having to replay the mission. A .snm file has been added to the main folder of the mod, which contains a number indicating what the next mission be.
If you go to main Original War folder and UserProfiles->[mod name, should be ArabianCampaign]->[user name]->CampaignSaves you will find your saves of completed missions, where every mission 11 save (something like Ar11-0$0$0$0$0$0$0$0$0$0$2$F.snm) will have another .snm file of the same name (Ar11-0$0$0$0$0$0$0$0$0$0$2$F.snm in this case).
You need to replace that file with campaign_hack_to_12.snm from the main mod directory, renaming it to match appropriate save ( like Ar11-0$0$0$0$0$0$0$0$0$0$2$F.snm in this case). After that you will see on mission selection screen, that the next mission after 11 (of that very save) is mission 12.
If you choose to play mission 12a despite that the save automatically "chooses' mission 12, you can replace the file in save folder with campaign_hack_to_12a.snm, the same way. That will take you to Legion branch.