I've often thought that the Armistice Line was pretty much close to where the Imperial border was when the Solomani Autonomous Region was initially set up. At war's end, the Imperium reabsorbed those worlds which had known it before the SAR while leaving aside those worlds which had only know the SAR.
We often forget to look at the timing of events. Admitted in 588, Terra hadn't been in the Imperium for 30 years when the Civil Wars kicked off and the Solomani Hypothesis wasn't verified until after Terra was admitted. Given that the Hypothesis had only been proven for less than a century before the Solomani faction at court saw it's power broken and seeing that the first 20 years of that period was spent in war and much of the rest in recovery, I really don't think racial politics played that much of a role at the time.
Afterward, sure. Both sides were looking for excuses and latched onto whatever worked. When the Sollie faction was sent packing, however, it was little more than a power play by Zhakirov. His mother had kept all the factions more or less in line, first by force and later by memories of that force.
As the "Man who followed the Legend", Zhakirov had to build a broad coalition of support and so opened his government to peoples of all talents. When the Sollies - many of whom were descended from the same clowns who kept the Civil War sputtering along - took exception to sharing power, he shut them out.
Having their "power broken" doesn't mean they lost all their jobs or were exiled off Capital and out of the Core Sector. It does mean they suddenly weren't able to rely on the Old Boys Network that had provided them with jobs, sinecures, and money for centuries. Remember, the SAR wasn't even set up for a quarter century after Zhakirov's marriage and the Sollie faction's "dismissal".
Given the issues the Imperium faced in the late 600s and early 700s, issues like a new dynasty settling in, the x-boat network being built, a hot border with the Zhos, and others, Margaret's establishment of the SAR looks like a wise, if short sighted, policy decision. Much like Artemsus "subcontacting" out various Pacification Campaigns to the Archdukes, Margaret "subcontracts" exploration, expansion, settlement, and defense along the Imperium's only open border while also giving potential malcontents something to do away from the Capital.
Her primary mistake, and the mistake of her descendants, was in not providing proper oversight for their "subcontractors".
In Wounded Colossus I took especial care to settle the Rim with an eye towards the long term. The Solomani were going to be hammered until they sued for peace, no armistice and post-war excuses this time. There would also be a belt of Switzerlands, Finlands, and the like between the two powers.
GDW wisely chose a "hot" border for the Classic Era OTU because it allowed certain adventuring opportunities.
