Eugh, I finally reached the Cult of Ulric White Dwarf Article. It has a timeline of Middenland that explains the whole Carroburg situation, and uh, it's more complicated than I thought. I'll try to explain it as best I can.
1124 IC: Count Mandred Von Zelt of Middenheim rallies the forces of Middenheim to fight back against the Skaven during the Black Plague, saving the Empire and soon after being elected Emperor after defeating the Skaven at the Battle of the Howling Hills.
1152 IC: Emperor Mandred is assassinated by Skaven, he has no clear successor so the Empire starts squabbling. Baron Reinhart of Carroburg claims to be Mandred's heir and declares himself Count of Middenland, so Baron Erich of Midddenheim invades Carroburg, kills Reinhart and takes the Elector Count title for himself.
1360 IC: Ar-Ulric Krieger enters a disagreement with the Count of Middenheim/Middenland and decides to move out to Talabheim where he is accepted. Since the Elected Emperor at the time was a Stirlander (Talabecland and Stirland have a long rivalry), Countess Otilla of Talabecland/Talabheim was miffed and so she declared herself Empress. The Ar-Ulric had the authority to crown the Emperor at the time, because Sigmar was crowned by an Ar-Ulric, so he supported her, which started the Civil War for real. Every Talabeclander Emperor afterwards called themselves Ottillan Emperors to honor Otilla.
1547 IC: The Ar-Ulric and Grand Duke of Middenheim reach an agreement, reconciling relations at the cost of the Cult of Ulric taking an oath of celibacy to prevent a rival dynasty forming. The Ar-Ulric returns to Middenheim and crowns the "Wolf Emperor" Siegfried of Middenheim/Middenland, which truly starts the "Age of Three Emperors" for real.
1550 IC: "Emperor" Siegfried leads an army to besiege Talabheim and take it over and is rebuffed. When he comes back he finds that the nobles of Middenland and Middenheim have renounced his rule and he's been replaced.
1812 IC: Baron Leonhard, brother to Count Karl-Heinz of Middenland/Middenheim, proclaims himself rightful Count and launches a brief but bloody coup. Count Karl-Heinz is ousted and forms a rival court in Carroburg supported by the court of Reikland with troops and weapons as long as Karl-Heinz helped Reikland with their own war with Wissenland.
Count Karl-Heinz then launches a siege against his brother Leonhard in Middenheim, trying to move through the tunnels in the Ulricsberg until Leonhard orders the tunnels closed. After the failed siege, Baron Leonhard is recognised as the Count of Middenheim/Middenland.
1865 IC: Jurgen, the Grandson of Count Karl-Heinz and currently ruling Carroburg, is rumored to be plotting to acquire the Electorship for himself from the Count of Middenheim/Middenland. The Count of Middenland invades Carroburg only to find it being defended by Reiklander troops, including a regiment of Greatswords that became known as the "Carroburg Greatswords" ever since. Jurgen the Baron/Count of Carroburg slays the Count of Middenland/Middenheim in single combat, reclaiming the title of his grandfater Karl-Heinz. In payment to the Reiklanders, Jurgen grants Reikland the former Count's wealth and estates, which might have included Carroburg. It's never really specified what he gave them.
There's a bunch of retcons all over the place, but I tried to organise this in a way that made sense and fit in general. It shouldn't be taken as gospel, but I merged the parts of White Dwarf and Heirs of Sigmar in a way that fit and explained things. The Age of Three Emperors was a goddamn mess.