I read most of this many years ago. I have a couple of notes to make.
1. Historicism goes back much more than 500 years. At least till 1200 when the Waldensians published their Treatise on Antichrist. It was also taught by John Huss and the Lollards, who were frequently in correspondence with each other. In fact the early Church writers taught an orderly fulfillment of prophecy.
2. Darby got his ideas of dispensationalism from Edward Irving who first preached on Dispensationalism on Christmas day 1825. He wrote in his Morning Watch magazine. The Irvingites then held annual prophetic conferences in Albury, Some of which Lady Powerscourt attended. She then held similar conferences in her home in Powerscourt in Ireland. Darby and Irving both attended some of these.
2a The lrvingites prophecied that the Rapture would occur July 15 1835. They gathered together to await the event. Basil Atkinson, Irvingism in its Rise, Progress, and Present State. 1836 Atkinson was formerly one of Irving's prophets.
According to H. H. Rowdon, The Origins of the Brethren, Darby didn't fully formulate his idea of Dispensationalism till 1841-2.