I’ve never seen any reference to it having been added later.  Mark believed Jesus had known he was going to be crucified (I think he probably suspected it would happen, may just possibly have provoked it), and all Christians believed he had risen from the dead.  Why would it need to be added later?  

In most translations, Jesus says he will be delivered into the hands of the Chief Priests and Scribes, then handed over to the gentiles for crucifixion.  It’s very specific.  More specific than you’d expect from a man given to allusory expression most of the time.  (Some translations simplify it to “into the hands of men.”)  I don’t believe Jesus said this, at least not in this way.  He would have been much more oblique.  Mark, following his theme that no one understood Jesus, says they didn’t know what he meant, even though clearly they know exactly what he means, and Peter upbraids him for making this grim prophecy.  It is one of Mark’s less convincing assertions, but he has to put that in there, or why would they have agreed to go with him?

It’s something written after his death, but not necessarily by Mark.  All the gospels have earlier sources that we don’t have now.  But none of them have anything directly from Jesus himself, because he left no writings, and probably neither did his disciples.  

Bart believes Jesus was referring to an angel empowered by God when he talks about the Son of Man, but hard to say why he’d think an angel would suffer crucifixion and death at the hands of mortals.  Whatever Jesus really did say, if we had exact transcripts, we’d still be arguing about what he meant.  Mark was not wrong to say that people had a hard time understanding him.  He was just wrong to think he wasn’t one of the people with that problem.  😉  

I doubt very much Mark didn’t have written sources.  Nobody wrote anything about Jesus’ life in any language for over three decades?  Unlikely.  But because his gospel is the first of the narrative works that survived, we can’t read any earlier ones to see who he was borrowing from.  Paul was not a storyteller.  So the speculation as to Mark’s sources is necessarily–speculative.  Some of those speculations, from real scholars, can be very involved–and very convincing.  Until you read a different one.  

If he wrote it all himself, he was a genius.