How did Jake lose his sight?
When he was younger, he and his friend both got drunk at a party and got into a car accident while trying to drive home. His friend was uninjured, and appears in the episode "Becker: Blind Curve (#2.11)".
What are Linda and Margaret's jobs?
Margaret is a nurse who manages Becker's practice and occasionally provides home care to patients. Linda assists with routine duties around the office, such as filing. She also once mentioned that she reads to the kids who visit the office. Becker once mentioned that he owed her father a favor, implying that that is the only reason he hired her.
How did Reggie/Chris become owners of the diner?
Reggie's father opened the diner and she inherited it from him.
Chris became the proprietor after Reggie left town