Even if 57 have the privilege to vote, I suspect that quite a small fraction of these usually visit the site on a day-to-day basis. You can see that quite many have their last visit weeks or months back, and most of these visits are probably not related to moderation. This is probably the reason why temporary drops in activity by a couple of the most active users (along with an unusually large inflow of poor questions) can lead to long review queues. Unfortunately, it seems like we have lost active/serious users at almost the same rate as we are gaining new ones.
Actually, after a quick count, I see that 17 of the 57 users haven't visited the site after 15 feb this year (if I counted correctly). If you would look at activity only during the last week about 10-15 more would probably be lost. So it would seem that the group of active users that potentially could participate in day-to-day closing is about 25-30. The "real" number of probably lower, since I suspect that some users almost never look thatat the review queues. These are the users we need to target for more effective community moderation though.
In either case, I agree that the moderation is a problem. My activity at the site decreased during the autumn/winter 2016 (I still visited several times a week though), and I sensed some problems with the community moderation when I visited the site. Everybody that visits/uses the site clearly need to keep an extra eye at the review queue, and users that cannot vote can still flag stuff as a way to moderate the quality of the site.