This is the most ambitious and effective of Diabelli's Op. 29 guitar sonatas. It also threw me off when I was studying it because it's the best example of an exposition that presents a Theme 1 that never comes back in the recapitulation.
Of course it didn't take long to notice that if your Theme 1 is a mere ten seconds long DO you bring it back? Theme 1 spans the mere space between 0:14 and 0:24 in Dylla's performance. In the score we're talking about a theme that is no more than eight measures long and, right away, moves to a transition. For months as I listened to and studied this piece I struggled to shake the idea that Theme 1 somehow included everything after those opening eight bars. Yet by 0:38 we're clearly setting up a dominant pedal for the arrival of Theme 2, which arrives at 0:46 and totally sounds like it's really Theme 2. Diabelli runs with a Group II in the dominant key that features a fulsome Theme 2 and a long Coda. Theme 2 comes back to start the recapitulation process at 5:05 in the video (measure 72 in the score). Everything in the exposition forward of Theme 2 comes back more or less on schedule in the recapitulation but we never get Theme 1.
If in the case of Sor's Grand Sonatas we saw that heavy intra-expositional repetition and development was a potential reason Sor resorted to truncated or recomposed recapitulation, in the case of Diabelli's Op. 29 No. 3 sonata, the most plausible reason we have for his truncated recapitulation is simply the brevity of his opening theme. If you don't bring back Theme 1 because it was played twice across the repeated exposition and was the basis for most of the development section then you could just go into Theme 2 and recapitulation the dominant key material only from the exposition. Charles Rosen observed decades ago that this would "count" as a proper recapitulation in sonata forms from the period and it seems as if Diabelli agreed!
Of course even in this, the finest of the guitar sonatas Diabelli wrote, we could propose he took the lazy way out. Why? Because he recapitulates his Theme 2 and 3 materials that appeared in the dominant by simple transposition. Of course "simply" transposing everything that was once in the key of C major into F major on the six-string guitar is a gruesome and demanding process. Let's cut Diabelli some slack here, okay? I know someone who wrote a guitar sonata in F minor for the fun of it and that person doesn't take a dismissive view of Diabelli opting to do a fairly simple transposing recapitulation. How many who would regard this as a shortcut have done it themselves? Diabelli may really have been a mediocrity and a hack but in this sonata he overcame his lesser composerly habits and wrote what I regard as one of the pinnacles of the early 19th century guitar literature. I actually like Diabelli's Op. 29, No. 3 more than any of Sor's Grand Sonatas. If you can get over the historical/scholarly consensus on what an uninspired hack Diabelli was and give this F major sonata a listen (particularly if someone like Marcin Dylla is playing it) you might find you actually like it. And even if you don't ...
I would propose, to keep my discussion of this sonata brief, that the best case study of a "Type 2" sonata for guitar would be Diabelli's Op. 29, No. 3 in F major. It also happens to be the best thing Diabelli wrote for the guitar that I've ever heard. This could be the one sonata Diabelli wrote that briefly lifted him beyond the designation of "hack" he's been saddled with (and not without cause!) by so many music historians.