There is no anatomical way a coroner could know a woman had an abortion 12 weeks prior. She could have had D&C after a miscarriage or for retained placenta after childbirth.
Frost deduces that the killer broke the glass in the door on his way out of the house, rather than on his way in, because broken glass hadn't been trampled into the carpet on the stairs or in the bedroom. But the police were alerted to the murder by a priest visiting the house, who must have had to walk over the same glass to find the victim.