I have previously on a couple of occasions mentioned the upcoming Karl XII : kungamord (Charles XII : regicide) by Cecilia Nordenkull. It has now been published. I don't exaggerate when saying unequivocally that it's the most weird book on this subject that I have ever encountered.
Her theory goes like this: Frederick of Hesse had been plotting against Charles XII for many years. In October 1718 he managed to convince Charles to visit Armfeldt's army outside Trondheim. When the King was well on his way Frederick made for Stockholm as quickly as he could, launched a coup and took power together with his wife, the King's sister. On carrying out this coup they claimed that the King had been killed (their plan was to have him assassinated during the journey to Armfeldt). It all went well and and even an envoy from the Czar arrived to congratulate them on their accession to the throne.
Meanwhile, the King had managed to avoid all traps and returned to his army in late October. Frederick, who also was back, found himself in a bit of difficult situation. First of all he needed to make sure that Charles didn't find out that he had been deposed. Secondly, he had to convince Charles to go "deep undercover" - Fredrick could not risk having someone finding out that Charles wasn't dead. Thirdly, he needed to kill Charles.
Then about a month passed, Charles never realized that he had been deposed and none other than the conspirators found out that the King was still alive. Now came the time to kill him, but how?
Well, Fredrick came up with a very complicated plan. He set up a battery across the river (some 700 meters from the Swedish trenches), borrowed some British officers and soldiers from admiral Norris, added a few trusted Germans and waited.
On the evening of 30 November the opportunity presented itself. The King had arrived at a gathering point behind the Swedish trenches, There he was approached by a young officer who had been ordered to carry a lamp. This young officer did not know the purpose, but when he reached Charles he gave a signal with his lamp. The battery across the river fired canister and Charles and the young officer were hit by many balls. Both men were killed on the spot.
Some of the conspirators now sprang into action. They carried the King tightly wrapped in his own coat to the trenches without being discovered and then put the King on top of the outer wall. Just as they were finished Lt. Carlberg appeared. He saw the King and joined the group of men. After some time one of the conspirators exclaimed: "The King has been hit!" and then everything proceeded as told in Carlberg's testimony.
So what's the evidence Nordenkull cites. Well, first of all a famous painting by Gustaf Cederström - dated 1897! It's not clear how Nordenkull thinks that Cederström knew the correct circumstances...
Another piece of evidence is a newspaper article from Stamford Mercury, which describes the visit by a representative for the Czar to Stockholm. Unfortunately for Nordenkull the relevant page has been mislabeled during scanning. It's quite obviously from November 1720, not November 1718.
What I have described above is just the tip of the iceberg. It's an amazing book - for all the wrong reasons. Even more amazing is that the author has been invited to a seminar in Halden - https://ostfoldmuseene.no/hendelse/historieseminar-1718/