Your analysis suggests that Gosho Aoyama has a range of options when it comes to introducing new characters or utilizing existing ones. By incorporating one-off characters, characters from longer cases, or those connected to recurring elements, he can keep the story fresh without running out of ideas. Additionally, characters that are somewhat independent from the main plotline can be reintroduced when needed to fill specific roles.
You also mention the possibility of characters who haven't appeared in a while making a comeback, such as Shiho and Mary, and even characters who have appeared in minor roles in the past. This approach can help maintain a sense of continuity within the series.
Furthermore, you discuss the concept of characters appearing briefly in a single panel, and how that might still count as an appearance. This level of detail showcases the depth of your analysis and understanding of the series.