Well done for doing this pretty epic and rather scary* walk! It's a little surreal just to think this wilderness exists so close to a built-up area simply because the canal cut it off,but it was fascinating to see. There's a chap on YT who's filmed this walk called "Bee Here Now" ; all his vids are worth a look especially the one where he makes it past the flooded bit of the Wapping Tunnel into a hidden jungle. Two other chaps also did Stanlow and went in and up almost everywhere - there's a 30 min vid on YT. They think the big house you went in was a combined police station and house for the PC.
I bet one could get away with living somewhere on Stanlow for years like in Blyton's "Secret Island" because so few people ever go there due to the walk,(except those guys on YT who snuck over at night in a dinghy).
The Echo article is actually good too - you said you couldn't find the abbey ruins but they have some pix of them there. Mount Manisty looks intriguing but I don't suppose there's anything built there,is there? You can't really tell from that drone vid on YT but it seems unlikely.
*especially as you had to walk back in the dark!
Dave.