1011
1012
Henry III (OTL Henry II, the Saint) was an obvious choice, but the nobles of HRE and the clergy liked him and in case of an election he would win but surprinsingly the emperor Otto recovered and was annoyed by this move. Gathering an army and with the backing of the pope Otto III attacks Henry III
in a small plain battle near modern day Unterthingau. The odds where even but Otto III was the better general and he defeated Henry III. Henry was then brought up in chains to Otto III who ordered his execution. Otto add to his personal titles the title of Duke of Bavaria
Succession to the title of King of the Romans and Holy Roman Emperor was not technically hereditary, although there were several dynasties that occupied the positions for several generations. Before OTL Henry II, the HREs held enough power that they could designate their successors. Henry II could not because he died childless. Otto and Henry were actually relatively close; in OTL, Henry was coming to Otto's aid when he died in 1002. The formal Council of Electors, the German princes who selected the Emperor, was not formed until the 13th Century. This is the primary reason why the first few HREs were able to select their own successors. In OTL, Otto selected his older cousin, Henry. In TTL, he has a son, Constantine, who he would likely choose to be his successor. The unofficial electors would most likely approve Constantine once he came of age, and the early tradition of the Emperor choosing his successor would continue for longer.
However, considering the above, the electors would not have chosen Henry to succeed Otto before his death, and almost certainly wouldn't have approved as Emperor anyone other than Constantine. This assumes that Constantine, or more likely his Regents, could hold onto power in the Empire long enough for him to come of age. Your events of 1012 are unlikely due to this. Perhaps a scenario in which Herman III, Duke of Swabia, comes into conflict with Otto over the attempted departure of Swabia from the Empire in 1012 instead.