In 1825 Several people in VDL knew her, and knew that Thomas and Catharine were NOT married to each other. (the prying eyes, and the letter writer, and Thomas, and of course Catharine) . Thomas died early 1829, and they had never married each other, BUT she was his principal beneficiary, so the impediment to their marriage was something HE understood and did not offend him, and the impediment still existed when he was dying.
Hence, I suggest that Catharine learned of the death of her first husband (not necessarily with surname McNally or variations), shortly after Thomas died (Jan 1829) and before Frederick Von S arrived in VDL (Aug 1829) otherwise she would not have been free to be courted by him, and afterall by 1829 EVERYONE in VDL would have learned of her lack of marriage to Thomas, and prying eyes issues would have arisen again, to cause the 1830 marriage to be challenged.
ANY SUGGESTIONS to follow up, as I have Not got any clues as to how to find the death of that earlier husband.
JM