Totally irrealistic, Dan.
An earlier alliance with Frederick III of Habsburg would change nothing since this emperor had almost no means and no power.
Frederick could not decide alone to make Charles king : he needed the approval of a majority of the imperial Diet. And the majority opposed the overambitious Charles becoming king in the HRE.
Charles was unable to beat the swiss, si he would not have defeated a coalition of swiss, lorrains and other allies opposing his ambitions in Lotharingia.
Well, the crowning was scheduled to happen, until Frederick III fled, apparently due to his anger over Charles' haughty attitude. This would seem to indicate that Frederick did, in fact, have the power, or had secured the power at the very least.
Also, the alliance I mentioned, was OTL; and, you are right, Charles was unable to overcome his opponents. However, in an ATL where he had better relations with the Habsburgs, things could well have done differently. In addition to the rebels of Lorraine and the Swiss (largely spurred on by the King of France), Charles also had to deal with Sigismund, a Habsburg, who was angered by Charles refusal to sell him back lands in Alcsase. Better relations with the Habsburgs would had stopped that latter conflict, and may have helped his overcome his opponents.
So, as you can see, its not unrealistic at all. Possibly a bit unlikely, knowing what we know of Charles' character, but not unrealistic.
But, very well, if you still disagree with that: A different sperm reaches the egg that would be Charles the Bold. This alternate *Charles is a poliitcal genius, as well as being compitent on the military field. He manages to play England, France and th Habsburgs off one another, secured a Kingdom for himself, and manages to produce several doughty sons to carry on in his stead.