I won't attempt to defend these points with correlations and markedness of oppositions, although in principle one could do so (Hatten 1994: 59-63 and 281-6, where I argue that the abnegational move underwrites a new expressive genre in both literature and music). Returning to m. 6, the Bb-F that I have considered an incomplete German augmented sixth has its own implications, stemming from the other plausible interpretation of the chord as a root position Bb triad, or bVI in D major. Beethoven cannot resist playing with this potential, both glancingly in the development, and more decisively after the intrusion of the X motive in minor just after the start of the recapitulation. This time, F-natural appears in the piano as well as the cello, and resolves to the Y theme in Bb major.
Having contextualized the meaning of the theme in m. 7, both with respect to the opening dramatic gesture and with regard to the consequences of the thematized resolution of m. 6, what more might one say about its inherent expressiveness as a motive/theme? Its contour is similar to one used in m. 6 of the Cavatina of Beethoven's String Quartet in Bb, opus 130, which I interpreted as a "tragic reversal" (1994: 213). But the context is quite different, and the result is affectively worlds apart. In the trio, the hopeful ascent is indeed reversed, but not negated, since the drop is enfolded within the harmony of V7 and is further resolved by step to complete a safely consonant triadic outline that could be reduced as 3-5-1. Yet the descending interval from 5 to 7 does have a certain yielding quality, like a graceful acceptance of boundaries, in its reversal of the initial ascent from 3 to 5. In turn, that tucked-in drop could be heard, in conjunction with the positive arrival 6/4 and pedestal dominant pedal, as somehow reassuring in its balanced motion. By comparison, nothing seems likely to stop the unbalanced additive sequencing of the X motive in the opening until it encounters the silence that anticipates a surprising and parenthetical F-natural.
Still other interpretations of the Y theme that might arise from chains of connotation or intermodally gestural considerations would include the trope that emerges from the "potential" character of the Y theme over a dominant pedal, as well as its "aliveness"-- both are supported by the evolving treatment of the 2-bar motive, which, though closed in itself, is treated rather "openly" as far as phrase extension through imitation, sequence, and fragmentation.