Does the doctrine of the separation of powers allow for an act of mock clemency in which a member of the executive branch interferes with ongoing court proceedings and commutes a defendant's illegal sentence to prevent its review and significant reduction by the courts?

Does the doctrine of the separation of powers as derived from the text and structure of the Constitution of the United States allow for an act of mock clemency in which a member of the executive branch, on his or her own initiative and against a defendant ’s will, interferes with ongoing court proceedings and commutes that defendant ’s illegal sentence to its functional equivalent in order to prevent its review and significant reduction by the courts? When a president or governor, on his or her own initiative and against a defendant ’s will, interferes with court proceedings to the sole detriment of that defendant by commuting that defendant ’s illegal sentence to its functional equivalent, do the courts need to place this defendant in the same favorable position as if the president or governor had never interfered?