The MP5K shown is stated as impossible for civilians to get, however many clones of the MP5K exist with full auto sears or registered receivers - and the operational briefcase to match them are in the hands of civilian collectors.
Treasures found in international waters are owned by the finder, so involved could only be charged with tax evasion.
Diana picks up the gun from the briefcase stating that it is an MP5K modified to shoot through a briefcase. The MP5K actually requires no modification if placed in a factory HK operational briefcase for operation.
The audio of Peter's voice clearly sounds dubbed when the shot changes between Peter's close-up and the photo of Richmond's building during the briefing scene.
When Mozzie first spoke to Neal over the radio he was still outside the building with Peter. He didn't insert his earpiece until he was locked in the holding room.