>> in my personal experience, "Professor" is used in some North American academic institutions as a lesser title only for those who do not hold a doctorate
Sorry, but I just find that very hard to believe. It really would surprise me, at any reputable college or university. It seems at least possible that you misunderstood the title and drew the wrong conclusion, or that the people at the academic institution in question were just mistaken in their usage, especially if it was a small college or a community college.
It's also possible that a private high school, as opposed to a college or university, might call ordinary teachers professors, just out of custom, but that's a completely different context.
In any case, as Ralf points out, none of that is relevant to JeffreyJohn's example, where the person is clearly a professor with a doctorate, so 'Prof. X.' is clearly correct.
However, it's also true that in English-speaking universities, being a university dean or president is normally a full-time job, so although it's often held by someone who has previously been a professor, they might not retain that title on taking the higher administrative office, unless they continued teaching students and doing research as well. So perhaps I should have added that administrators are often just addressed as 'Dr.':
Dr. Fred Smith
President, University of X
Dr. Maria Jones
Dean of Graduate Studies
That's actually what I would expect to see under a signature on a diploma, so in that sense, I agree with JeffreyJohn's gut feeling.
So the short answer: In English, either 'Prof. X' or 'Dr. X,' depending on context, but never *'Prof. Dr. X.'