The reason I added the Kakashi part is because it's assumed, it's not fact, which when you typed it, you typed it as if it's 100% fact that's how he got it lol. Also no, Itachi gave him all of his powers (As stated by Madara) I can link you if you want, as for the Itachi unlocking his Mangekyo, it's a mystery still since he said Shisui died a silent death then he followed that up with "I helped him accomplish this" all in a different set of dialogue, it's now unclear if he killed him or not.Well thats what i just said aswell:
Kakashi gained trough blaming himself
Sasuke after itachi's death ( stated by madara ) itachi just gave him amaterasu to try to kill madara if he would approach sasuke.
Itachi MOST proberly after shishui sacrificed himself and gave his eye to itachi. ( MOST logical theory for now since he stated he didnt kill him )
Shishui i just doubt he killed a friend, maybe trough hard training aswell. his method is most mystery for now.
Madara and Izuna i already said trough training where they were competing against eachother.