So I ended up doing something I swore did never do, which is watch “ending EXPLAINED!!!” videos and read threads about the movie on Reddit. None of them had an interpretation that I liked, but they all had enough pieces to put together into what I think is the most plausible interpretation:

Jack’s feelings of guilt over Madeleine’s death (and presumably his affair with June), combined with desperation over his show’s poor ratings, slowly overtook him over the course of the show. The guests that were supposed to be sensationalist and attention-grabbing started to implicate him more personally, mentioning his wife, threatening to reveal his affair, reminding him of the ritual he’d done at the grove. He hadn’t fully believed in the supernatural aspects of it, but as Carmichael told him, he was easily suggestible.

After the hypnosis, it all came to a head and he had a psychotic break, killing everyone on stage. It culminated with his stabbing Lilly, at which point he found himself back in reality, on a stage in an empty studio surrounded by everyone he’d murdered. The “it is done” message at the end suggests an attempt at ambiguity: if you don’t accept the supernatural, then it’s just the host of a show killing his guests on live TV. If you do, then the deal he’d made back in the grove had finally been settled: he was now world-famous, and an episode of his show had more viewers than Carson.

I still don’t think it entirely works (not least because it doesn’t explain how he could’ve burned Carmichael to death), but it does seem to fit most of the points the film keeps banging on repeatedly. And explain how this could’ve been a live broadcast but still been obscure enough that some people had never heard of it.

I guess I do have to give the movie a little more credit than I did initially. It seemed to be so broad and so insistent on being an over-the-top funhouse that I didn’t see any room for ambiguity, or any way the events could be interpreted as anything other than literal.