Hi Mark,
First of all, well done reading this complex
monogram. You have rightly assumed that it hides more than just the personal name. If an office is combined, I think episkopou (or indeed, archiepiskopou) is the most probable guess. Another possibility would be eparchou, but I can't think of any names that could go along with that.
If we read episkopos, that leaves M and/or X and A, and possibly N - and here we run into problems, because there are multiple options, as any of the letters of episkopou is 'allowed' to be added. So indeed,
Markou is possible, but Mariou, Marianou, Maurikiou, Eukarpiou, Kyprianou all fit the
monogram as well.
You say it feels that X and N are unaccounted for, but we do need to assume the X when M and A are read. And there is no need to read archiepiskopou, although that is very much possible. If we read archiepiskopou, then the possibilities for the personal name would increase even more, because we can read any name with an A and any letter of the
monogram.
In the course of the 7th century, the block
monogram is pretty much abandoned in favor of the cruciform
monogram. I think one of the reasons must be that complex block
monograms like this one almost always yield multiple readings.
Regards
Gert