I understand the frustration of people who end up answering the same question over and over again, and definitely don't want that to be the reason people get tired of participating. However, the same concern about attrition of our current user base applies to the potential narrowing of a new user base. As with identification questions, these leaf issues are frequently the reason why people come here in the first place. To them, it's a new question, pertains to their own plant, and wants it's own attention from the community, and we don't want people to feel unwelcome. You people have spent years trying to create that perfect balance between knowing when it's better to answer, link, or duplicate, hence the reason we're here in the first place!
I'm far from qualified to help, but I'm wondering if there's a way to supplement the definitive answer you're in the process of building. It's based on something I found at Cooking:SE, and I wouldn't be surprised if it was born out of the exact same type of frustration. (@Stephie, I know you spend a lot of time doing great things over there, so maybe you could correct me if I'm wrong.)
Is there anything to be said for creating lengthy tag descriptions, including references to certain questions on the site where people can look?
For example, at Cooking, the food-safety tag has a long description designed to incorporate everything your definitive question would.
It has categories, each with lists and links, including:
Overview of food safety
Fundamentals, including top 5 things you need to know
Precautions to take
The answer to the basic type of "most asked question" from the community
Links to helpful outside resources
Links to further reading/most frequently asked questions on the site.
The format is legible, and the actual categories could be tweaked to work for our most common leaf or plant problem tags.
If the OP hasn't used the tag, we could add it once a diagnosis has been made. Even if the tag's there, that doesn't mean they've "studied" it, so we could add a comment linking to the tag description, as we do now for possible duplicates or things like query results, as Jim Young suggested. We could even add a message nicely asking the user to read it.
In a perfect world, people will look at potential duplicate questions as they write there own, or even after. In the same perfect world, people will read a novel-length tag description. This is not a perfect world, so I'm not saying this is the best suggestion, just a thought.
I know that adding tags is something we try to avoid, but we have a spider-mites tag with no wiki or description. If anyone agrees, and feels like writing it, we could use it as a test.