Much of the pagan stuff looks to be based on James Frazier's work, which has been criticized as inaccurate. Considering that the pagan cult was made up by the original Lord Summerisle, who belonged to the Victorian Era, that source would indeed have been the best work on paganism available to him at the time.
The fact that the religion was made up in Victorian times but based on and portrayed as a set of much older traditions is also a dig at Gardnerian Wicca, which was very popular in the 1970s
Late in the movie, Willow and MacGregor try to give Sergeant Howie the Hand of Glory, which would immobilize him - If they believed it would work, it would have jeopardized the whole plan by keeping him in the inn instead of letting him chase after Rowan. However, their real goal was the exact opposite - scaring him away from the inn so he could continue his search. Word had probably spread that he'd been in the library, so the plan somewhat hinged on him having read what the "hand of glory" was. Though it probably would have worked either way - if you were feigning sleep and someone put a candle made out of a dead guy's hand next to you, you'd probably run away whether you knew what they were trying to accomplish or not.
The restored edition adds a lot of motivation for Howie. Not only is he engaged, but his fellow police officers mock his virginity constantly as well as his faith. The villagers are pressing all of his Berserk Buttons, and the only reason he is able to withstand Willow's seduction is because he is engaged, not because he's a virgin and/or religious.
A possible motive for Willow's attempted seduction is to verify that Howie is a virgin and therefore an appropriate sacrifice (they can't afford to screw it up). Since it's impossible to know for certain that he's a virgin (rather than just trying to pretend for his wife, and willing to put up with ribbing for it), they make sure nobody on the mainland could find out and sic the best seductress they could find on him. If he resists, his virtue's true.
The child torturing the beetle comes off like gratuitous weirdness, but it's a Genius Bonus — a "beadle" is an old term for a policeman. This greatly strengthens the idea that the beetle is symbolically representative of Howie: the closer he gets to finding the truth, the more trapped he becomes.
Invoked with Howie warning Summerisle that he will be the sacrifice for next year. Another would be that Howie's disappearance will not go unnoticed by his police force. They could send in an investigation team to find out what happened, and all the townspeople will be in deep shit.
It actually enters Nightmare Fuel territory, when you think about the entire scheme to locate a virgin policeman and bring him over to the island. Is it a coincidence that he's so smug about Howie's absence not being noticed? It's not impossible to have a former islander in the police for and/or a Corrupt Cop who would cover up Howie's tracks for cash. Plus, can you entirely trust Summerisle when he says that it was the worst apple harvest since his grandfather's time, when Howie points out that the island isn't suitable for apples? For all you know, there could have been other (covered-up) harvest failures in the past, each one followed by an unfortunate virgin policeman being burnt alive.
Another fridge horror moment involving Rowan Morrison. Of all the policemen whom they could have tricked into being the sacrifice, they specifically required the only one they knew was a virgin. And Summerisle himself says "A small child is good, but not nearly as good as the right kind of adult." Had Howie accepted Willow's "invitation" and lost his virginity, he would have ruined his suitability for a sacrifice, and the islanders, discovering that their sacrifice was ruined, might have actually sacrificed Rowan instead. If Howie were to live, it would be at the cost of Rowan's life.
There's another possibility for the islanders' Plan B in case the Howie plan didn't work: Lord Summerisle himself. His Oh, Crap! expression when Howie warns him that he'll be next might not be because he didn't consider that before, it's because he didn't want to be reminded of how close he came to being the sacrifice himself.
If you take Lord Summerisle's statements at face value, then it's easy to take the islanders' plan as an act of desperation, out of simply not knowing how to handle such a massive crop failure. Contrast this with The Wicker Man (2006), where it's made clear that what happens to Edward is something that's happened many times before, making you wonder just how many poor guys got needlessly burned alive just because the island's bees produced a cruddy harvest the previous year.