It's not primarily a question of political judgment but of a personal one.
Bismarck had been Wilhelm's Minister-President for the past year, and had worked for him in other capacities before. So it's reasonable to suppose that he had learned a bit about the latter's mental processes.
So if he thought there was a real possibility that Wilhelm would fall in with the proposed reform, I don't see any obvious reason to doubt him.
And Bismarck, who knew his man (as by your own admission you do not) clearly wasn't prepared to bank on it.
If the success or failure of the Congress had been determined purely by the decisions of Wilhelm I, that argument might well stand. As it is, the argument doesn't (in my opinion). It would be determined by the behaviour of various figures (most prominently Franz Josef and Wilhelm I), how they reacted to each other, the demands of various figures and how they impacted the decision-making of other figures, how persuasively various figures communicated to each other… Bismarck could have known Wilhelm I perfectly and he still wouldn't know the result of the Congress without a keen understanding of how nations viewed their long-term interests, and even with such an understanding it would be difficult to predict the interrelated actions of those various figures. His treatment of France and Russia is sufficient to convince me that he didn't have that keen understanding.
As for personal knowledge of Wilhelm I, by my own admission I indeed do not. But even there, I'm not sure I'm convinced that Bismarck knew him so excellently. When Wilhelm II was growing up Bismarck had far more than a year with him and was in far more of a position of authority and knowledge, but he misjudged Wilhelm II's character so badly that he was sacked and ended up a bitter, respected but powerless old man.
If you are simply arguing that Wilhelm's presence in Frankfurt does not guarantee success, I have no quarrel with that. If FJ pitched his demands too high, he could indeed have yet snatched defeat from the jaws of victory. However, my impression from your messages is that you view failure as a virtual certainty, which afaics the evidence does not support.
I am arguing both. My argument that failure is a virtual certainty comes from the contradicting interests of Austria and Prussia, not from the particular figures involved. I only started speaking of the particular figures involved as a counter-argument to the idea of Wilhelm I personally causing German unity and Bismarck personally denying it, first proposed by Alpha Trion and later taken up by yourself.