Maybe it's my ignorance of SE beta protocol/requirements, but I fail to see why we really need to define a strict "target audience". I mean, I understand the need to have a clearly defined statement of purpose, rules for inclusion/exclusion of topics etc. That in itself directly affects the target audience and will prompt some people to participate more than others.
But the idea of defining the site as catering to one specific group or another, as if it was even possible to mould the existing community into something else, strikes me as a bit odd.
Many of your examples seem to revolve around question levels and whether we should be a "beginner" or an "expert" forum. Why in the world do we have to pick one and how would such types of questions be mutually exclusive? This is what tagging, clear titles and browsing is for: people can find the level they like to work with and stick to it. But as @Ali pointed out, "level" is quite a vague concept and in many cases, it is more about people's strengths and weaknesses, and how they complement each other: this is precisely the point of JLU (one might not know some obvious piece of grammar, but be able to weigh in on a fairly advanced daily-life expression, while a book-learner might give great tips on formal use of some forms, but need pointers on conversational skills etc. etc).
At the end of the day, this might be an issue if there was any actual pressure to keep one or the other out of JLU. But there just isn't. I have yet to see one single, well-formulated question of any level get downvoted or even ignored. Beginner questions that were closed, usually were because of how they were asked ("teach me all you know about...", rather than "what is the proper way to use X and Y"...), or because they failed very short of the minimum quality standard (straightforward dictionary look-up questions). As many have pointed out, there are a lot of great beginner questions and they've received a lot of great, thorough answers, and no criticism that I can see.
In conclusion: I think we all agree that even more topical questions, beginner or expert, would be great for the site. But getting more of these is a matter of getting a wider audience, not one of weaning our current one.