Leave is interchangeable with let when followed by alone with the sense “to refrain from annoying or interfering with”: Leave (or Let ) her alone and she will solve the problem easily. When he was left (or let ) alone without interruptions, the boy quickly assembled the apparatus. The use of leave alone for let alone in the sense “not to mention” is nonstandard: There wasn't any standing room, let (not leave ) alone a seat, so I missed the performance.
Other substitutions of leave for let are generally regarded as nonstandard: Let (not Leave ) us sit down and talk this over. Let (not Leave ) her do it her own way. The police wouldn't let (not leave ) us cross the barriers. See also let¹.

Did you mean let off?