Knight says that Gibbs and Ducky knew each other twenty years. Actually, they met in 1980, established in the Season 18 episode Everything Starts Somewhere (2020), before Gibbs became an agent with NIS, the predecessor agency to NCIS.
Jonesy is a homeless veteran living on the street, with little to eat. He was clearly very grateful for the sandwich they gave him at NCIS but left a sizeable piece on his plate. In his situation, he would have eaten everything and would probably have licked the plate clean.