I am wondering about the faked powder traces on the faked suicide, the killer sprinkled some powder onto the victim's hand - but - wouldn't a gun firing create a specific pattern of powder residue which would be hard or impossible to fake?

Actually, this is overlooked during the final scene. As the conclusion draws near, Oscar Finch asks Columbo if there was powder on the gun hand and how did it get there if Staplin did not shoot himself. The final clue is supposed to be so decisive that the explanation of faked powder marks seemed unnecessary.

The killer could have fired a second shot into a pillow by pressing the finger of the victim onto the trigger of the gun placed in the victim's hand to create a genuine powder trace.