How is it out of place in verse 9?
It would be odd for Jesus to use the phrase "to the paralytic" in the presence of the person he was referring to, while speaking to the scribes: "which is easier: to say to the paralytic, your sins are forgiven...". Rather, Jesus would have used the words recorded in Matthew: "Which is easier: to say, 'your sins are forgiven...'. The phrase belongs in v10 where the narrator uses it: "he said to the paralytic...".
"Pick up your mat" doesn't make sense placed before "and walk":
"...or to say, rise, pick up your mat and walk?".
But it was clearly part of Jesus' command in v11: "rise, pick up your mat and go home".
It would be very odd for Mark, if he was composing the story, to use two phrases first in their wrong context and then in their correct context! It's much more likely that his source used the phrases correctly and that Mark inserted them incorrectly by mistake or in trying to expand his source.
In which case why would we look any further for Mark's source, than Matthew?
Edit: Actually, Mark at this point is following Luke not Matthew, so his source is therefore Luke, who added the details about letting the man down through the roof. But Luke has copied Matthew's wording for the dialogue in the passage.