Unless a Justice also writes their own opinion, their joining in the decision does not necessarily indicate they agree with every sentence and phrase of the commentary. Justices often have their own and sometimes independent and contradictory reasons for joining the majority or minority opinion. They often do not comment on those reasons.
It is unfortunate that you resort to belittling the controversy as a "conspiracy theory" in order to smear the reputations of the people opposing your comments. It is an incontrovertible fact that President Chester Arthur appointed Horace Grey to the Supreme Court of the United States. It also an apparent fact that Chester Arthur's father was a British Subject with British citizenship when Chester Arthur was born in the United States. Under the interpretation of the Constitution that Chester Arthur was a natural born British citizen due to his father's British citizenship, Chester Arthur was not eligible to the Office of the President. In the event that Chester Arthur was discovered and found to be ineligible to the Office of the President, he could have been removed from office by the U.S. Congress, and all of his appointments and other acts would have been null and void as if they had never happened. Consequently, despite the fact that Chester Arthur was already out of office and deceased, Justice Grey's acts would also be null and void as well, and he would have to be removed from his office in the Supreme Court of the United States.
Given such circumstances, Justice Grey's unnecessary additional commentary about the natural born citizen clause in a case not addressing the question was in effect self-serving and an obvious conflict of interest, whether or not anyone can prove he was aware of the conflict of interest. So, no conspiracy theory is required to observe the obvious facts of the circumstances.
You said it yourself: in the Court's opinion, Justice Gray added "unnecessary additional commentary." An examination of the Opinion quickly reveals that Sections II and III, which contain the bulk of Justice Gray's analysis and in your eyes are nothing but dicta, are long and detailed. Yes, joining a decision does not indicate agreement with every "sentence or phrase," but we're not talking about a word or even a sentence. We're talking about pages upon pages upon pages of dicta. Your reply is greatly weakened by just how long Sections II and III are. If the other five Justices felt that Sections II and III were unnecessary, they would've written separate or concurring opinions to express their discontent with Justice Gray's "unnecessary [and extremely long and detailed] additional commentary." They did not.
Given such circumstances, Justice Grey's unnecessary additional commentary about the natural born citizen clause in a case not addressing the question was in effect self-serving and an obvious conflict of interest, whether or not anyone can prove he was aware of the conflict of interest. So, no conspiracy theory is required to observe the obvious facts of the circumstances.
Well, it wasn't obvious at all to President Garfield, who chose Chester Arthur as his running mate. Nor was it obvious to Winfield Hancock, President Garfield's Democratic opponent in the Election of 1880. (By then, mudslinging had already become common in elections.) And last but certainly not least, it wasn't obvious to the 4,446,158 Americans who voted for the Garfield-Arthur ticket, or the 214 electors who cast their lot with the Garfield-Arthur ticket.
So obvious to you, yes, but no, not obvious to others.