Again on the argumentative structure of Pārthasārathi's Śāstradīpikā

So, points 1 to 3 and point 7 are definitely a pūrvapakṣa. Instead, points 4 to 6 are agreed upon by the siddhāntin as well. Points 8-9 are the transition to the siddhānta, opening at point 10.

Why did Pārthasārathi choose such a complex argumentative structure?

It is typical of him that he lets his own view merge into an uttarapakṣin's one (so, for instance, in the Nyāyaratnamālā, while dealing of śabdabhāvanā). He could thus have meant 1-3 as an objection becoming gradually more plausible. Then, 4-6 are definitely his view. 7 continues one of the elements of the 1-3 objection and 8-9 express it fully. 10 rejects it and the following part of the text expresses Pārthasārathis' view.