Although Sherlock's reasoning was complete and reasonable, only Iver knew that Sherlock's answer had to be wrong.
That was because he was missing one crucial piece of information, but Iver alone could not tell Sherlock about it.
Following the clue concerning Iver's grandfather, Sherlock had traced things to the Noble Red Society. Adding in the Pelican Bar, the Giant Remnants, and the Smuggling Case, he had naturally concluded that Iver's parents had both been killed by the Noble Red Society.
The deduction was perfectly logical.
However, during the Dream Realm ritual, Iver had interrogated the Curse Master "Aziz Ben Abdul" as a Deformed Limb Demon and learned from him that he served Professor Moriarty. He was the true master of the Iron Hook Demon, and the one who had killed the entire Alexander family.
But he was not a Demon Warlock of the Noble Red Society.
He was not even from Avalon, and had nothing to do with the Giants or the Stibnite People... He was a Parthian.
This was information Sherlock himself had mentioned to "Fox" during the Dream Realm ritual.
At the time, Sherlock had only known that "the mastermind behind the scenes eventually left the country, with either the Horus Empire or the Parthian Ancient Kingdom as his destination." But Iver had seen that man's appearance with his own eyes, and even learned his distinctly Parthian name.
--In other words, the Alexander family, as the founders of the Scaleless Hand, was connected to the Noble Red Society, and the Noble Red Society certainly had both the motive and the ability to kill them. Yet the person who ultimately carried out the plan was a foreign Curse Master hired by Professor Moriarty.
The side with the motive had not acted, while the side that had acted had no