"not merely did" is correct and usable in written English.
It is typically used to emphasize that an action or situation is even more unusual or unexpected than previously indicated. For example: "Not merely did he arrive on time, he was actually early!".
Not merely did they face being pitched into deep poverty overnight but they would endure the stigma of being labelled a "scrounger".
"As I say in this new book of mine, not merely did I learn to kill," he told Sheldon Hackney, who was then chairman of the National Endowment for the Humanities, in a 1996 interview about "Doing Battle".
As for Britain and France, there have been frequent public acknowledgments of the obvious fact that not merely did they not fight for Czechoslovakia in 1938, but that although they declared war on Germany when Hitler attacked Poland in September 1939, they did virtually nothing to help Poland militarily.
"Not merely did Howard seek to eliminate the private landlord: he eliminated the temptation to increase density in order to raise land values.
In his reinterpretation and eulogy of Howard's work, Mumford (1938: 397) for instance stated that: "Not merely did Howard seek to eliminate the private landlord: he eliminated the temptation to increase density in order to raise land values.
Therefore not merely do effort and labour cease, but also hatred and bitterness.
One thing that Clegg has been highly successful at doing is ensuring that he is not merely doing things but seen to be doing them.
These oligopolies, which are actually sanctioned by the S.E.C., didn't merely do their jobs badly.
France did not merely make errors of judgment.
Don explains he is not judging; he merely didn't know Pete was miserable.
