Plothole?
Why go through the trouble of actually stealing the real painting?
* Pretend like you stole it. (just set of the alarm so Keegan knows something was up at the museum)
* Give fake painting to Keegan
* Carlos sees its a forgery (even no need to have the father play the appraisal guy)
* Carlos kills Keegan
* End of story.
What does it matter to John Travolta that Carlos gets pinched afterwards with the real painting?