If an owl had attacked her she'd be shouting and screaming
An owl attack has been likened to being whacked with a baseball bat..so, no, it's not actually that likely she'd have been "shouting and screaming". She may well have been giddy and disorientated from it.
The owl theory is highly plausible given the evidence:
Talon shaped wounds on her head (that look identical to wounds seen in actual owl attacks)
Three microscopic feathers from an owl's foot. One in hair she was clutching in her hand, two in hair on her head
Blood drops on the front drive way
Pine needles on her hands and legs indicating that she'd fallen over outside
Blood smears with her fingerprints on the front door consistent with her shutting it while bleeding
Evidence from her footprints that she was bleeding before she got to the staircase
Blood spatter going up the stairs rather than down
Evidence from three experts that her wounds are consistent with an attack from a particular breed of owl (forget what it's called) that is known to live in the area and has attacked other people
The evidence supports the theory that she went outside, got attacked by an owl, got knocked off her feet (which would account for her bruising), had to physically wrench the owls feet off of her scalp (explaining the ripping), ran inside, slammed the door (leaving bloody fingerprints) and then fell backwards while trying to get up the stairs.
People tend not to scream when they are having to take action (like running or fighting off an attack) as they have to conserve energy so it's unlikely she was screaming loudly outside. And if she'd received a blow to the head, that and the biggest panic of her life probably stopped her thinking clearly and all she wanted to do initially was get the fuck away from the owl...possibly not realising how badly she'd been hurt.
Whatever hurt Kathleen ripped her scalp without bruising or damaging her skull - this kind of injury is almost unheard of when one person bashes another person on the head, but it's classically seen with bird attacks.
The owl theory is actually plausible when you look at the evidence.
Oh, and if there's no evidence the fountain was on in December then there's none that it was off either. Maybe they liked the tinkly sound and had it on whenever they sat out there.