Regarding the letter you quoted, can you really deny that she is not discussing Mansfield Park? Certainly the topic of ordination is related to the topic of the first part (Austen's writings) rather than the second (hedgerows in Northampton). The fact that Austen was in the midst of writing the novel, combined with the fact that the subject of Edmund's ordination is a significant plot point in the same argues strongly in favor of the interpretation that Austen was referring to Mansfield Park when she mentioned ordination. I must, unfortunately, confess that I find your argument that Austen fails to describe the actual ordination of Edmund forecloses the possibility of ordination being a theme weak. After all, in none of her novels are the marriages of the protagonists described, but one could hardly therefore claim that these novels are not about marriage. My point is simple--because the most reasonable interpretation of the phrase about ordination in Austen's letter is that she was referring to her current project (Mansfield Park), we would be remiss in refusing to consider the theme of ordination in Mansfield Park. As I wrote before, it is not an either/or thing. That ordination is a theme of the novel does not preclude the existence of other themes. That the entirety of the novel is not an allegory of the BCP ordination rite no more makes the novel not about ordination than the fact that Tolkien refrained from turning the Lord of the Rings into an allegory (a la Lewis' Chronicles of Narnia) makes it not a deeply Christian work. Ordination is a significant theme of Mansfield Park regardless of what Austen had to say about it, but I maintain that her letter shows this to be deliberate rather than accidental. The topic of ordination is the source of the dramatic tension between Edmund and Miss Crawford. Their disagreement as to what ordination means and how an ordained man is to live and act is the recurring problem that makes a union between the two uncertain. If they can eventually agree or compromise, they will marry, if not, they will not. Because Edmund's ordination is the thing that prevents his certain union with Mary Crawford, he, and therefore the reader, must figure out what ordination means, and whether it is sufficiently important to sacrifice Miss Crawford for it. The theme of ordination then intersects beautifully with the theme of acting. Is Edmund Bertram a clergyman, or is he just pretending to be one--is he just playing Anhalt? It is the choice to give up Mary Crawford (not the external one forced by Maria's adultery with Henry Crawford but rather the internal one to see her for who she truly is) which is his true ordination. It is at this point that he truly assumes his priesthood because it is at this point that he has rejected acting (judging external performance) and embraced identity (judging internal character). In contrast, ordination is not a theme in Sense and Sensibility notwithstanding Edward Ferrars' clerical state because his ordination is not the source of dramatic conflict in his minimal character arc. Being a clergyman for Edward Ferrars is simply his role in the story just as Mr. Collins or Mr. Elton are clerics in P&P and Emma--if they were lawyers, their characters would be unchanged. However, if Edmund Bertram were not a cleric, then Mansfield Park would be a completely different story. Mansfield Park, therefore, is about (amongst other things) ordination.