Heres my take: Ruby, her dog, was laying down in front of the basement entrance. Amanda, on the phone and not likely paying attention, enters the room from the "north" room we see in the chair photo. She trips on ruby, ruby out of surprise goes running into the chair at the table, knocking it over, her phone goes flying and in an attempt to recover, she puts her hand out to brace against the wall by the basement door. She misses her mark in the midst of her fall, and winds up shifting all her weight meant for the wall, towards the stairs, sending herself careening downwards. Her forehead hits the piggy bank and she falls again. She starts bleeding badly. She struggles to stand after the fall but manages to do it eventually, and stands at the foot of the stairs before succumbing to the blood loss and lying on the ground.

This is the only way I personally think the accident theory could work. She was high and disoriented as it was, you add traumatic blood loss to the mix and you are going to fall over very quick.