The stuff about the temperature sort of tallies with what I see in brewing records. In the 1820's, Barclay Perkins were pitching at 63º to 67º F after which the temperature rose to around 80º F. Though the stronger Stouts were pitched much lower, at 58º to 59º F. By the end of the 19th century, their pitching temperatures were down to 60º to 62º F.
“Pantologia: A new cyclopaedia Vol IX”, 1813, (doesn’t have page numbers)
Using sour beer for preparing finings has one pretty obvious disadvantage: it's a good way of introducing an infection. You can't help wondering how often such finings buggered the beer in addition to clearing it. Later in the century brewers moved over to using other dilute acids in which to dissolve finings.
I'm not sure what to make of the final section about the strange beer engines. Is it to be believed? Was this really common practice? Certainly a couple of decades later brewers were advised not to send out their Porter mild, but to always blend it themselves. Mostly to reduce the possibility of the publican adulterating the beer.
And the flavour descriptions: mild Porter was "bitter mawkish" and stale Porter "in some degree acid". I think I can understand what the second means. But the first? The word mawkish has fallen into disuse and doesn't really say a lot to me.
That might be it (at least for a day or two) about Porter. But you never know, I might find something else I just have to share.