1. Jotaro being hit by a regular human being with no special capabilities whatsoever is DEFINITELY PIS, Jotaro being damaged by Stands mean that they scale to Jotaro's Durability, if I have a character take SEVERAL hits and have a fight with a planet level character for multiple chapters, as well as having fought other planet busters in the past, but if they are not implied to have gotten stronger and City busters have also damaged said character, it means they have only been damaged due to PIS, Jotaro has been damaged by The World, Killer Queen, Crazy Diamond, Magician's Red (Who even has a Large Building level feat), and Anubis (Which could harm Polnareff, who took hits from Magician's Red and DIO), for example.
DIO had regained confidence in battling Jotaro after taking Joseph's blood, up until this point, he had been afraid of Jotaro's ability to move in frozen time and resorted to using knives, using a cop to make sure that Jotaro is dead, and even after that using a sign to make sure that Jotaro is definitely dead, and DIO even tried running away to escape Jotaro by stopping time and grabbing onto a car so he could get Jotaro to hit him into Joseph, so he could take his blood and become even stronger than before, he only started toying with Jotaro because he believe himself to be all powerful and unbeatable at this point, instances in which Jotaro is damaged by significantly weaker beings should not be deemed as anything more than PIS as they contradict his most common feats
2. Well, with Joseph being on par with the Pillar Men and Caesar being on par with Joseph, they should definitely scale to them