Bear in mind that what defines circumstantial evidence, as opposed to direct evidence, is that, by definition, such evidence is consistent with more than one conclusion. A fingerprint at the scene of the crime is consistent with it being put there during the crime, or at some point prior to the crime, or even planted later. So too with the uniqueness of Chief Justice Roberts legal position.
Obviously, he could have been persuaded that the mandate was unconstitutional after oral argument, and sometime later he came also to be persuaded that he had a purely legal duty to give a “saving construction” of the statute, for reasons that have nothing whatsoever to do with the political implications to the Court of invalidating the President’s “signature” legislation by a 5-4 vote of Republican-nominated justices. But if the strength of the legal reasoning of an opinion is relevant to assessing the motivation of its author – as has been so long and loudly done for all five justices in the majority of the remedy portion of Bush v. Gore, then it seems pertinent as well that Chief Justice Roberts was the only person in two years to look at this legal dispute and reach this legal conclusion.
I wonder if there has ever been another case in Supreme Court history to be decided by a single justice’s legal theory that had not previously been held or advocated by any other person in the United States before he announced it. One even he had not deemed persuasive until some time after his initial conference vote (a fact that no one seems to dispute). Whether or not it was political, the uniqueness of Chief Justice Roberts’ opinion may well be unprecedented.