"If Johnson's claim is based on negligence, is the defense of assumption of the risk applicable?
No, if it appeared that there was no other practicable way of getting out of the lot before Monday. 
No, because Johnson was confined as the result of a volitional act
Yes, if a reasonable person would have recognized that there was some risk of falling while climbing the fence. 
Yes, because Sales Representative, as Car Company's agent, waited for help. 