Logline structure and plot aside I think there are some fundamental problems with the concept.
1 - plausibility; to actually be able to threaten, torture and ultimately kill someone or a group of people the scenario needs to enable the mechanism that allows the bad guy to do this. As such you've added in the most recent drafts a generic description of the characters are being kept as prisoners. This is the same as in so many other movies that have a similar premise and as a result doesn't sound interesting. What is unique about their entrapment?
Just a quick thought; If you don't want to involve any super natural themes, could they have a movie contract at the end as in the surviving writer is guaranteed to sell their script? The antagonist can then use their career aspirations and greed against them.
2 - motivation; the antagonist seams ruthless for almost no reason, normally an antagonist's motivation can be omitted from loglines, but as the genre in this instance requires it, his motivations needs to be crystal clear, compelling and unique. It's his motivation that will determine how bad an antagonist he will be and therefor how interesting the characters fight to survive will be.
If these fundamental problems can be solved after several iterations of the logline then this could make for an interesting concept if not and after many re drafts the logline is still not working maybe it won't make for a good concept.
I don't want to sound mean or inappropriate just trying to be helpful in my (all to well known) direct way.