This didn't seem all that confusing to me. The owner's ghost wedding itself was a little high-concept, sure, but the actual mechanics aren't too complicated. It's actually remarkably similar in motivation to the clinical trial story: someone doesn't like the real world, and they believe they have a way out: a ghost marriage where the participants are together forever. So they hold a lot of these ghost marriages (where the bride is, idk, killed and then temporarily kept animated via ghost energy or whatever, who cares), and by doing that they collect wedding guests. Some of the families seem to be in on it, especially the one of the dead guy who says the bride was "sent to" the ghost. Those who can see the real ghost wedding are extended an "invitation"—to what? Why, to the owner's own ghost wedding! Which requires you become, well, a ghost first.
In the end, the deaths of the participants rack up police suspicion, and the venue owner is forced to tip their hand a little early. Despite not having enough wedding guests, they decide to go ahead with their own ghost wedding. The mechanics of a ghost wedding we already saw earlier: it involves exchanging one person's head onto another. So the owner supplants their own head onto that of another bride, and then calls on all the forces of Hell in order to supplant the heads of the groom and the wedding guests, to have a ghost wedding. This of course kills everyone involved. One interesting thing to note is that they are not supplanting a ghost wedding: there were, originally, an actual human groom and an actual human bride (you can see by the altar, this isn't the ghost wedding venue in the basement). This also fits, however, with the entire venue hall being haunted, as we saw earlier when the deuteragonists went back to rescue their coworker, and with Nishi saying the entire wedding hall is "their playground".
One interesting thing of note: this does fit with the fact that Yume saying "it stinks" will cause it to stink more. After all, people who see the ghost weddings for their reality are extended an invitation... therefore, the threat is dire only if you say something silly like, oh, I don't know, "it stinks it stinks it stinks..." at an otherwise perfectly normal wedding convention. Best to keep quiet, eh?
Moral of the story is the same as usual: if you think you've managed to find an escape route from reality, you haven't. It's just that this time the mangaka is exposing some of the, idk, "fake happiness" around weddings? Or maybe saying they're overly idealized. And also, we see Hama going back to save people, which is some nice character development.