I have a similar theory. I think it is most likely that she tripped over the dog and fell down the stairs. The fact that the piggy bank clearly had a mark in the wall were it had been pushed into basically confirms it. The staircase also looked very poorly made with a big area that was easy to fall into. I also know Calgary very well and have seen the kinds of drifters they were describing. The idea that one would enter your yard for anything other than bottles is pretty absurd to me and no way would some random one sneak in and kill you without a motive.
I know her family is very convinced she was killed, but it seems very obvious it was a freak accident. The ones to blame though really are her husband and neighbours. The fact that they all heard the dog yelping panicked and did not call the cops basically killed her. Especially the fact that her husband was on the phone with her and it cut off suddenly. Then he could not get a hold of her for days and just thought she was busy. He talked to her everyday. He should have asked someone to check on her. Very likely it would have saved her life.