His name is never stated, so he could be Oliver's kid brother. The real Oliver might be home sick, or he could be sitting this year's contest out so his little bro can have a shot at winning.
The date of Nigel's recruitment and on the package Hadden sends for him are from 1978, yet Nigel is clearly from our own time (clothes, hairstyle, references to the Middle East and TV celebrity chefs, etc). Hadden's note from the package suggests it was written after their phone conversation, even though he refers to the package having already been sent that morning during that discussion. Much of this can be reconciled if Mr. Hadden is actually observing Nigel by mystical means from 1978, and leaving directives to Hare and Crow that they can carry out in 2008.
The documents Nigel stole depict a supernatural Swirly Energy Thingy very similar to the one in Ganwulf's tomb. Letting Nigel get interested in the Lost Crown, and recruiting to investigate the ghosts that could lead him to it, was just a sneaky way to gain access to one of those.
Nigel hears a distressed cat's ghostly meowing from inside the chest in Harbour Cottage, yet the person arrested for the catnappings ( Mr. Gruel) didn't have a key to that place. Morgan did have one, so could have used the Cottage to hide the kidnapped cats until they were weak enough to handle easily and she could sneak them into the old Net Hut. One of the cats was left in the chest too long, and suffocated, creating a meowing ghost, exactly like the one that he later hears behind the door to the Nightmare Room.
After Nigel occupied Harbour Cottage, she couldn't use it anymore, which is why Mr. Tibbs was strong enough to escape her at the Hut. Then she framed Gruel to deflect the investigation when Nigel and Lucy uncovered evidence of her crimes.
Normally, they wouldn't bother to start plaguing a would-be Crown-hunter until the trespasser got dangerously close to its hiding place, which is presumably why they hadn't killed Hardacre years ago. However, the Hadden photos of the Chasm had enough of a supernatural aura of their own to worry the Agers: they know the other ghosts in the region have information about the Crown's location, and the photos' presence implied that this treasure-hunter might try to communicate with those ghosts, so the Agers tried to head off Nigel's investigation by scaring him off in Hardacre's guise.