Hi!
One can always misunderstand on purpose. That episode was before the King decided not to go to Frankfurt.
You sure of that? According to Crankshaw [1]
"On and on [Bismarck] hammered until, just before eleven o'clock, the king burst into tears and gave in. Bismarck himself, hurrying off to impart the news to Count Beust - - - was so worked up that he slammed the door behind him, pulling off the handle in an access of nervous violence. Later, when he reached his own apartments, he picked up a glass jug and hurled it to the floor."
Crankshaw goes on to describe the conversation with Beust in which Bismarck threatened to summon Prussian troops to Baden-Baden and surround King John's residence unless he were on his way back to Frankfurt by tomorrow morning.
But even if you insist on dismissing all this as exaggeration (you have evidence for that?) I still don't see how it affects my essential point, that Bismarck had no cause for concern about the King going to Frankfurt unless he thought that agreement between Wilhelm and Franz Josef, allowing the latter's Reform plan to go into effect, was a real possibility. A visit to Frankfurt which proved fruitless would not harm Bismarck's position; if anything it would make Wilhelm all the more dependent on him.
[1] Bismarck, Ch X.