The thing is, it's becoming increasingly apparent that Cummings is not a genius. He's a baldy little virgin who thinks he's cleverer than everyone else. Every briefing from the "senior Downing Street source" (aka Cummings) about how they can do this, that or the other, is comprehensively dismantled in minutes by actual lawyers.
Whatever he comes up with for the 19th, the fact remains that the Benn Act is the law. In the event that he tries to subvert it, the court will order that someone else can sign the letter, and it'll be incumbent upon Parliament to remove him from office. If he refuses to send the letter, there'll be more resignations on the Tory side, and it'll be Johnson, Cummings and the dickheads in the ERG stood trying to take everyone on.
This won't go the way the bald virgin has dreamed it up in his wank fantasies.