The sheer physical strength of the dog to be able to hold up April while swimming against a raging current to hold April in place just long enough for John to finally get there imply a dog of great physical power. That kind of stamina did not bely an aging dog with a heart condition.
To be fair, Thelma Baird had turned into a whacko, sex-crazed woman before the end. After the funeral, Elly gave her ex-husband a dinner where he spent the time hitting on Elly and promised to make the Pattersons a regular visit. Naturally, he was never seen again.
The ravine did appear once before in a comic strip where John imagined a map of the neighborhood. Lynn Johnston based the house on her house in Dundas which had a ravine in the back. In mind it was always there, but she occasionally has a problem that her readers are unable to read her mind. This and the magic toy boat in a backyard where April plays on a regular basis were the two drop-ins that made the story go. However, to Lynn’s credit, she did introduce the idea of April getting out of the backyard, Farley’s age and the idea that only Farley was taking care of that kid well before this story came. That level of story setup is not usual for an author accustomed to dropping things in to make the joke.
As for the rescue, that was so “Timmy in the well” complete with the stretch out to get her that just barely makes it, Lynn was clearly watching rescue movies or TV shows to get that story.
Elly’s father was modeled off of Rod Johnston’s mother, who had a stroke right in front of Lynn.