It may also be useful to keep in mind that the scissors incident was speculation - it is mentioned in the novel that nobody was certain if it actually happened, like most of the other Radley stories. In fact, one neighbor, Miss Maudie I believe, voiced her opinion that it wasn't true at all. Then again, she was a firm supporter of the Radley's and their right to privacy and to not be the center of the gossip circles. Miss Maudie reminds the reader that people should not be judged unfairly - just one theme of the novel.
Although Boo's father "saved" him from juvenille detention, Boo's sentence was far greater than it would have been had he simply served his time. Boo did stab his father with the scissors. His father was domineering (and there are suggestions that he was emotionally abusive). Boo stabbed him because he was angry. As to how effective it was--who knows. It got him temporarily removed from the house, but he was locked in the courthouse basement instead. Mr. Radley eventually took him back, and one can only assume that life in the Radley house continued much as it had before the incident. Boo was thirty three at the time of the scissor episode, and already mentally crippled for life.
Boo supossedly stabbed his father in the leg with a pair of scissors when he was a teenager, then returned calmly to clipping articles in the paper. There had been some friction between Arthur ("Boo") and his father, the result of a teenage prank in which a group of bad boys Boo had fallen in with locked up a beadle in the Maycomb County Courthouse outhouse. Officials wanted to send Boo away but his father had convinced them to let him stay home, where he remained in seclusion for some 15 years.