i heard that another possibility was a woman who was a piano student of his for a short time. her name was josephine von brunswick. she married joseph carl deym who happened to be related to the viennese emperor, making her a countess (i believe). anyway, she also had a sister.
the modern day version of her would be isabella rosellini ? was she in the movie Immortal Beloved? I haven't seen the movie, so i was just googling.
because beethoven was scared to use her name in the letter (ommitting a title or addressee), i think she was possibly married already. he started lessons with her in march 1799 and again in october. did she marry over the summer? i don't know. just guessing.
caroline van beethoven was carl (nephew) beethoven's wife. alexander thayer, the great biographer of beethoven, actually visited every person of importance - which included caroline van beethoven. it would be interesting to read the recollections.
did you know that otto jahn, who had undertaken a task like thayer's (recollections and letters and research), but abandoned it basically turned over his gathered material to him. alexander wheelock thayer wrote 'the life of beethoven' in three volumes. the first volume took seventeen years, and another thirty-two to finish volumes two and three. he never really finished it, and others completed the task of finishing and translating it into english.
mr. thayer was born in massachusettes in 1817 and received a liberal education at harvard. after graduation, he went on to take an interest in the life of beethoven and went to europe in 1849. he spent two years researching in bonn, berlin, prague, and vienna. he came back to america and worked for the NY tribune for a couple of years, then returned to europe in 1854. he studied precious documents in the royal library at berlin, unearthed much in bonn, and returned to america again.
maybe a careful reading of his works would shed more light?