Yep, the mixture of reasons is quite convincing. I'm sure some places in that age were overriden for way less serious stuff. But then, I'm definitely not an expert in history...
And, of course, if one wants to indulge in some cold-blooded power politics (as would also have been realistic) Coralynth might well be concerned about Sorraine taking over the entirety of Ohnar West and want to grab a bit of it for themselves to off-set the growth of their most likely war partner.
