PLYMOUTH, Mass. – Mark Pothier was wrapping up the young-adult phase of his music career, including a 1983 stint as keyboardist for the then-synthy band Ministry, when he took a $5 an hour job for the Pembroke Reporter, a recently launched community paper along Boston’s South Shore. Pothier rose through the ranks of the umbrella Memorial Press Group (MPG), a chain of suburban newspapers, and in 1986 was named executive editor of the flagship, the Old Colony Memorial in Plymouth.