Merton seems very Orthodox in Seven Story Mountain, did he remain so the rest of his life? I know he was a Peace Activist and the like, but there is nothing wrong and inconsistent with that.
That's a little unclear. I am not exactly an expert on Merton's theology, so I hesitate to speculate. There have been various rumors. Merton had a slightly checkered past. He had fathered a child out of wedlock prior to his conversion. His experiences at Cambridge and Columbia University placed him, shall we say, left-of-center.
In The Seven Storey Mountain, he attributes reading Etienne Gilson's The Spirit of Mediaeval Philosophy as providing some of the inspiration for his conversion.
Assuming he was genuinely inspired by Gilson, that would tend to place things in the orthodox camp.
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Etienne Gilson[Atyen´ zhElsON´], 1884–1978, French philosopher and historian, b. Paris.
He taught the history of medieval philosophy at the Sorbonne (1921–32) and then took the chair of medieval philosophy at the College de France. In 1929 he helped found the Pontifical Institute of Medieval Studies at Toronto, Canada. Although primarily a historian of philosophy, he was also one of the leaders of the Roman Catholic neo-Thomist movement. He was elected to the French Academy in 1946. Among his works are The Philosophy of St. Thomas Aquinas (1919, tr. 1924); The Christian Philosophy of Saint Augustine (1929, tr. 1960); The Spirit of Mediaeval Philosophy (2 vol., 1932, tr. 1936); God and Philosophy (1941); Being and Some Philosophers (1949); and The Philosopher and Theology (1960, tr. 1962).
Some of his early work--Seven Storey Mountain and The Sign of Jonas, especially--are inspired. His later work was trash. By the late sixties he had taken a left turn and was digging into eastern mysticism; he also entered into an affair with the young nurse he had met during a stay in a Kentucky hospital. Since he was living alone in a hermitage in the woods by this time, he was able to arrange to see her frequently. He also partied a good deal with his New York friends. So much for the benefits of contemplative life...