I'm sorry to have to say this, but there is nowhere near enough ventilation.
The usual guideline is 1 square foot per adult, standard-sized hen of 24/7/365 ventilation -- best located above the birds' heads when they're sitting on the roost.
Chickens, like all birds, have extremely delicate respiratory systems -- remember the canary in the coal mine thing? -- and need a generous supply of fresh air at all times.
-7C, 19F, is not cold to a chicken because they have built-in down parkas.
I'm not knowlegable about chicken diseases and injuries so I can't help you with any ideas about what happened to your lost bird, but poor ventilation will certainly cause trouble over the long term.
Here's a great article on cold-weather chicken keeping:
https://www.backyardchickens.com/articles/cold-weather-poultry-housing-and-care.72010/