The War of the Imperial Succession

[Mestra]

I'm questioning why Mestra and Carpathia would instantly support the one faction that seems to have the least chance of winning.

Mestra and Angiris have at this point been allies somewhere around 700 years, give or a take. It would be likely then for the Imperium to support the faction with whom the treaty would be transferred to. Of course, this goes back to my original question about how that would work, and of course it would depend on what the treaty originally said (that part was never worked out). In any eventuality, the Imperium will likely not rush straight away but debate whether to adhere to the treaty, which it will do in the end.