I didn't pick up on the implausibilities in the Eurus backstory because of all the other problems I had with the season. First, Mary getting killed because of Sherlock being vindictive and it being done as the start of his redemption. Secondly, just how explicitly Culverton Smith was based on Jimmy Savile, which I thought was tasteless. And finally, because Sherlock getting such a loving brotherly feeling for Eurus, even though Eurus had killed several people in a gratuitously cruel way, was played as his complete moral growing-up, which upset all my "feeling altruism towards one specific person you like does not constitute moral awakening" issues.