Mark can play anything apart from metal and its variations. The difference between him and Robbie in this department is that no matter the style you know it’s Mark Knopfler whereas Robbie isn’t distinguished.
Yes, that's called a signature sound.
Question is, can he really play anything else but some Mark Knopfler ? We don't really know because he hasn't really showed anything else.
He tried some slide, but I found it a bit dull compared to other slide masters. I mean it wasn't bad, but it wasn't that great either.
He experimented a couple of things but again it was far from 'master or genius level'.
It seems to me that he is stuck with his style. Just like Slash, who is another favorite of mine. You can instantly recognize him, it's great and everything, but he is also stuck in his own style. It's not a bad thing, it's just how it is.
Robbie is on the same level as Richard. You give him anything to play, you tell him how it should sound, and he'll get the job done. Hence why session or studio musicians are highly demanded. They're very good at everything, not excellent at just one thing.