For had fallen in a when-clause, can the past perfect tense be replaced with the past tense?
Easily the most boring class was History of Magic, which was the only one taught by a ghost. Professor Binns had been very old indeed when he had fallen asleep in front of the staffroom fire and got up next morning to teach, leaving his body behind him. Binns droned on and on while they scribbled down names and dates, and got Emetic the Evil and Uric the Oddball mixed up.