Do they even attempt to explain Mrs. Reid's absence? Is she dead? If she's coming back, I might watch series 3. I started watching the show mainly because of Amanda Hale. Huge fan..
I am not sure how far you've got watching the series but she did not actually leave him. Rather she seems to have had a nervous breakdown which I think lead to her being sectioned. I am not too sure on the details anymore though. At the time grounds for a divorce included adultery, cruelty, or desertion, so there would have been no real legal basis for a divorce.
Yes she basically went insane and died. Also in season 4 Matilda talks to her father about her mothers mental issues and if they were similar to the woman they were watching out for who lost her son at the workhouse.