My first idea was that the raa in ramadaana can be read with a shaddah (a tajweed prounciation that is not consistently reflected in text). Reading this with a shaddah would automatically imply that shahru was not just shahru in an idaafah construction but rather shahrun with idghaam. This idghaam consisted of the assimilation of the noon into the raa' of ramadaan. Again these specifics are based on the premise that the rules of tajweed are not accurately represented in Quranic texts, and that this was a reading that most recitors did not take because of their traditional reading traditions.
Peace
This presumption was even unsound. Firstly the "tanween" could not be presumed on the first noun in construct state. Secondly "tanween" could not be presumed because of "single fatha" on Ramzana. You could presume "tanween" and shadda if Ramzan had "Dhumma tanween" whereby you could take it as adjectival phrase.
