That code-identicality alone doesn't necessarily mean much: lots of people come later and will use the public question/answer code in their own work. (Only together with the other pseudonym correlations, it means more.)
Right, it wasn't just the fact that the code was the same; it was that he initially posted those questions under his real name.
Absolutely none of this is evidence of his guilt, it's all just very loose circumstantial suggestion. If this is all it takes to convince people of someone's guilt in a huge criminal case, we're all doomed.
"Probable cause" is all you need for a warrant. With the warrant you secure the "beyond any reasonable doubt" evidence.
No, the financial transactions will prove his guilt. This is for finding him in the first place and getting the necessary warrants. Also, note that this evidence is for the indictment, which is what it takes to bring the charge, not a full statement of the case at trial.
Circumstantial evidence has sometimes been enough for "beyond a reasonable doubt" in many cases over the past hundred years. I guess we were doomed since before then?