The prohibition has never been on
moving the capital for strategic benefit—as you say, nearly every capital move has
some strategic benefit.
It is specifically on
moving the capital for the strategic benefit of recruitment closer to the front.
And, as was complained about a certain amount recently, it
is at least partly a crime of intent. The same action can be considered either definitely fine or definitely illegal, depending on what people have discussed and stated as its reasons. (Though most of the time in cases like this, it's somewhere in the middle...)
Given the situation you have described for Highmarch, I would be highly skeptical of any capital move that happened in the near future. Frankly, I'd be highly suspicious of any capital move that happened less than a year after the creation of the realm unless it was to move the capital
away from a front that had suddenly come to it (due to regions being taken by enemies).
Consolidation of military gains is something that most generally happens during peacetime. There is an important line to be drawn here between this sort of thing and the secession that led to Highmarch's creation in the first place, as a) secession inherently creates more opportunity, and b) secession removes regions from a realm, which can quickly alleviate the serious pressure that being too large can place on a realm's regions. The pressures of being far from the capital tend to build up more slowly, and are by their nature not shared equally across the realm.
...I hope that sounds more coherent to you than it did to me...
