The efficacy is not accurately measured by the size of the Unanswered category. Yes, there are about 48K questions there, but only 34K actually have no answers. The rest have at least one answer, which in many cases fully deserves an upvote, but does not have one yet. And among the questions with no answers at all many received helpful comments; unfortunately there is no way to obtain a count of those where the comments helped.
Let's count the questions that truly sank without a sound: no answers and no comments. Exclude those that were somehow rejected (closed or downvoted). We are left with ... 9461 questions, out of the total of 262K questions currently on the site. Not that bad.
To your second question: I took the reputation of the askers of the most recent 10000 questions; focusing on recent questions allows to reduce the effect of postfactum reputation changes, and also gives a better idea of the current state of events. Out of $10000$ questions:
- $82$ were asked by users with reputation $10000$ or more
- $183$ were asked by users with reputation $5000$ or more
- $955$ were asked by users with reputation $1000$ or more (called "established users" by SE)
The median was $61.5$.
So, less than $10\%$ of askers were "established" by the SE standard. The reputation of the rest is presented in the histogram below:
Then I did the same for $10000$ most recent answers. Out of these:
- $700$ were posted by users with reputation $100000$ or more
- $2995$ were posted by users with reputation $10000$ or more
- $7140$ were posted by users with reputation $1000$ or more (called "established users" by SE)
The median was $3734$. (The author of a randomly chosen answer is more likely than not to be able to vote to close.) Established users, who ask less than $10\%$ of questions, answer more than $70\%$ of them.
This time, the histogram is based on users with rep under 100K: