There is an infamous typo in Sigmar's Heirs that talks about the date of Marius's death. Apparently two numbers were switched or something, because every other source dates the battle against the orcs in the Third Battle of Blackfire Pass where Marius was slain all the same except Sigmar's heirs. It's only wrong in that one spot in the book so it's clearly a typo.
I just think that span of time is GW slacking. The fluff is so shotty as to who is running the show and who the heirs are. If I had to reason it out I know that the Leitdorfs are probably doing some infighting and the Alptraums are trying to seize power back from them.
An interesting story would be if Marius had no sons, or one that is not of age, or some other reason that would pass legit succession to his brother Hans Leitdorf. Hans could refuse to take the crown of Averland on the grounds he is a grand master and holy Templar of Sigmar (his order is extremely devout), so he might hand it off to another heir. The rest of the fighting could be between family members, powerful allies, the alptraum family, etc.
We'll never fully know the story until GW does something with it, which may not happen for a long time sadly, considering the rich history and the fullness of character behind Marius.
I say Hans becomes our Elector Count. One thing is for sure though, Averland has no shortage of Aristocracy to lead the masses.