Why do the characters commenting on Jonathan's past cases know more than they should? One of his fan club describes how he solved the crime from 'Jack In A Box', but I thought he hadn't told the public about that. And in another episode, someone praises his skills in solving the judge's murder from 'Mother Redcap', but he gave the credit for that to the policeman in charge of the investigation.
I don't think it was ever stated that he did not talk about 'Jack in A Box'. And the implication running through those scenes was that it was in Maddy's book (since the fanclub were getting everything from the book). So that would be my explination for that.
Jack Holiday's butler begged him not to tell anyone that his master was a murderer, and I got the impression that he agreed. Granted, that wouldn't necessarily stop anyone from writing about who killed Holiday, but it seems to be a needlessly cruel thing to do to corrupt the memories Rokesmith's family had of him. Although I guess it might have been necessary for him to do so for the sake of the justice system retaining its integrity.
I don't recall Holiday's butler actually asking them to keep it quiet — he admitted that he learnt the truth, but kept it quiet out of respect for his boss — but in any case, Rokesmith was found sealed in the bunker where Holiday was found; presumably the police would still have had to be called to deal with that body, and the truth came out from there.
Possible scenario: Maddy writes up all the mysteries, but changes the names and locations. Now, the scene in which the Jonathan Creek nerds discuss the Jack Holiday case does have them using Holiday's correct name, but it could be that in that particular instance they had deduced who the real identities of those involved, and name-dropped them in order to try and impress Jonathan.
Adam Klaus is played by an actor from Boston with a strong New England accent, but his sister Kitty has a broad Scottish accent. While it's not so far-fetched that one of them may have picked up a local accent upon relocation (and maintained it), can siblings' accents really be that different? And where do Klaus and Kitty come from originally?
Probably; it would depend also on how early the accent shift occurred and how hard they worked at it. It's also possible that Adam, being something of a pretentious twat, is the kind of guy who'd not only adopt a false accent but stubbornly maintain it around people who know full well that it's a complete act.
Adam Klaus' accent is almost certainly fake in-universe - just as his real name is Chester. The guy's a full-time fake. Plus, of course, Rule of Funny.
The Grinning Man
Surely the people being tipped into the water tank would have splashed water all over the bathroom? Or, in a more disgusting thought, wouldn't decomposing body bits from previous victims have washed back into the tub?
In the former case, finding water splashed around in a bathroom might not arouse suspicion by itself.
It might if the towel was still dry. "They disappeared at the point they took a bath" would seem to be a reasonable deduction, in that situation. Probably also why the original murderer went to some lengths to recover the soap.
The bathtub may get rid of the body, but what about clothes? Mina may have left hers in the bedroom, but surely some would have done what Joey did and got changed in the bathroom leaving clothes and underwear on the floor? - surely if they did that would have been a giveaway for what they were doing when they vanished.
Also, why would the guests (particularly the more recent ones) have a soak in a tub that had obviously never been cleaned in several years? You'd probably come out even grubbier.
Maybe Lance Gessler knew about the trap and regularly maintained and cleaned it (but figured it would be too risky to remove the bodies from the tank.)
Jack In the Box.
If the bunker wasn't intended to be used and "no one had been down here for years" why had large boxes full of useful supplies (soap, lightbulbs, loo rolls, matches, etc) been just left in there? How wasteful.
Amateur Survivalism. Survivalists know always keep a good supply of essentials. Jack and his wife most likely had the basic non-edible supplies (which are easier to get) but stopped before they bought food (which is harder to get - at least the stuff that will last years without going bad). Remember they did intend to use it in case of nuclear war. Jack and his wife didn't build it themselves and likely didn't have the simplest clue what they were even doing. They failed to realize that soap & matches go bad after a few years. (Matches need to be replaced after a few months.)
Also, it's directly stated that the bunker was commissioned during a time of great tension between the Soviet Union and NATO, but by the time it got to the stage it was things were much calmer. Presumably he bought them in panic and then, once he calmed down a bit or got distracted by something else, simply forgot about them.
Alan Rokesmith was wearing a leather jacket when he shot Holliday but had removed it when he sealed himself in the wall. How come no one found it in the bunker and got suspicious?
Maybe he took it off because he got hot with all the hard work of brick laying and already put it inside the fake wall before he gets in himself.
The Judas Tree
The ending of The Judas Tree. The killers get away with murder and no one seems to care! Okay, I can understand that Emily and Kim killed that mentally-disabled boy when they were teenagers and were let off scot-free. But what happened to them seems like Disproportionate Retribution: years later, to murder one in cold blood and frame the other for her murder. And even if you do believe that they deserved it, it's still rather off-putting that Jonathan and Joey apparently decide to keep quiet about the whole thing, especially since Joey was friends with Emily, and Jonathan was romantically interested in her (well, almost had a one-night stand with her at least).
Joey and Emily weren't friends, they simply met through Joey's blog and she'd agreed to try to help her.
Plus, the way it's framed isn't so much 'oh well, we don't really care that you murdered one of them and stitched up the other one' and more 'we know you did, but unfortunately we can't actually prove it'.
The Judas Tree. What the hell?
So much of the hidden mystery is dependant on the fact that the police are completely incompetent in every faccet of thier job. Why the hell didn't anyone do a background check on Emily to see if she had any prior criminal history? Wouldn't someone doing basic paperwork have noticed that Emily had been on trial for another murder? And looked up what happened to to the murder victim's family? Even though the husband had changed his name, someone might have looked into his history as part of the case. Even if the police decided it was unrelated and ignored it, there WOULD be a nosy reporter who would want to track down and interview the family of Emily's previous victim to get their thoughts on the current trial. (Especially the press of the Jonathan's Creek's world, who are regularly portrayed as insanely biased.) On finding out that Emily had a former murder case, SOMEONE would have tried to track down the other accused. Or better yet, realize that the victim of her last case had the same damn home address as the victim of the current case.
Also, any competent medical examiner, on noticing a murder victim was pregnant, would have done a DNA test on the baby to make sure it was the husband's. If it wasn't, then that might have bearing on the friggin' case. ("Hmm...maybe the husband was in on it...")
Secondly, why didn't the medical examiner do a simply tox test and see that the murder victim had a serious amount of tranquilizers in her blood? Far too much to even be walking, let alone struggle against someone pushing her out a window.
The body would have had rope abrasions on her wrists and ankles too, signifying that she'd been tied up.
Why was the wife so willing to give up her entire life, her career, her family just so that her husband could get revenge on two girls who may have killed a kid thirty years ago? How is she going to even leave the country? Use the dead woman's identity?
Also, the entire plan is dependent on the idea that, I don't know...Emily wouldn't just quit. No matter how good the money if I started seeing ghosts, having hallucinations and my boss was a bitch I would leave.
Beryl Carney's estranged daughter Hannah figures out that her mother is actually alive and masquerading as her twin sister Marella Carney within seconds of meeting her again just by intuition. Why then, were the dead Marella Carney's own family not able to deduce that it was not Marella who had survived but Beryl? Surely Marella's husband and daughter would have been able to work this out because they knew the sisters so well - there must have been differences between the two that the husband and daughter would have known about, especially if estranged daughter Hannah is able to work out what had happened so quickly.
Yeah, it's ludicrous to suppose that the Marella's husband and twelve year old daughter wouldn't instantly have known that Beryl was taking their wife/mother's place, but there is an attempt to handwave it when the butler says: "you were blinded by her love - for you, for Charlotte."