However, the theory raises more questions than it answers. The (temporal) Electoral title is linked to a specific province, not a family. That the title could shift between family branches within a province is certainly feasible - but between provinces not so much. If the Baron of Nordland was the heir to the Grand-Barony of Westerland (or vice versa), why should he inherit only title, not the land? Why does he not simply become Elector-Count of Westerland and Baron of Nordland in personal union? Why should Westerland/Nordland be an exception to this basic rule? Sure, political expediency might in exceptional circumstances require to split the title from the lands – but on several occasions? Without any reference in history? And if the buck had been frequently passed between Westerland and Nordland, why should it suddenly become a problem in 2301?
Of course, if we could find a source that mentions contemporary ECs of Westerland and Nordland, the issue would be settled beyond doubt.
Such is the problem with many of these contradictory background issues.
On the first count, perhaps we are wrong to assume the intrinsic link between 'lands' and an Electorship. Certainly there is a portion but arguably the greater importance is that the Elector's represent one of the founding tribes and a link back to Sigmar. In line with the thoughts put into the book 'Runefang' perhaps a great deal of the credibility of an Elector comes not only from holding an important demesne but that they also bear one of the great swords forged for the founding tribes chiefs. Considering the two fallen provinces, Drakwald and Solland, you have situations in which each is missing one of the components key to reinstatement, the Drakwald 's lands were so devastated and overrun by Beastmenthat it was absorbed into Middenland (by in large) despite it's Runefang still in the hands of the Emperor and Solland though also devastated by Greenskins of Gorbad Ironclaw they did not stay and occupy (unlike the Beasts) but had no Runefang. Thus if the Runefang is passed the to family with the closest relations to the original line that inherits the Runefang then perhaps the Electorship follows it. Thus at times the Prince in Marienburg bore the Runefang and when his line died the Prince of Salzenmund received it and also the title of Elector.
To further flesh out my theory, the line of Marius and Endal Princess lasts some centuries but as true lines do it dies out and Nordland and Westerland are granted to cadet families with the senior granted the Runefang and the Electorship and the other nominal vassal of the other.
The lack of references is to be expected from the very spotty background but during the Civil War years I think it likely that rival Emperors created a myriad of 'Electors' on a whim. Perhaps some mention of an Elector of Nordland are not a true Runefang bearing elector but merely creation of the Middenheim Emperors to bolster their claim to the throne.
Lastly 2301 was a problem because the plethora of outstanding and long held rivalries between previously warring factions. The Baronial title was dissolved as the burghers and minor nobles in Westerland had no interest in being ruled by a now alien and very recently hostile Prince. The title for Marienburg was retained but the Baron of Nordland had no direct control over the region. WHFRP 2 fluff suggests that the Nordland Electors, Gausser in particular are keen to reunite these lands they feel are rightfully theirs and even suggests that if Gausser were to retake Marienburg he would reinstall the Niske family in Salzenmund as his vassals. (From Shades of Empire if you are curious)