I heard the final chapter was supposed to include a summary of the whole book. What happened to it and who eventually wrote it?

You are correct. As explained in Chapter 53, Hemadpant had intended to write a summary to conclude the book, but he passed away before he could do so. His final chapter was found written on scraps of paper, but the summary itself could not be located within them. After his son, Gajananrao, brought this to the attention of other devotees like Babasaheb Tarkhad, they agreed a summary must be written. An announcement was even made in the Shri Sai Leela magazine, but the deadline passed without it appearing. Ultimately, the narrator of Chapter 53, feeling unequal to the task but praying for divine grace, took on the responsibility of composing the summary.