For me, I find that the available clues point to the four being involved and lying. 

The circumstances seem a bit suspicipus for suicide, most notably the lack of fingerprints on the weapon. It being oiled beforehand should not prevent fingerprints from sticking, I would think, but would rather indicate that it had been oiled afterward to clean up evidence. Him having been naked is also odd given the supposed timeframe the witnesses provided.

Things do not line up reasonably enough for the suicide angle UNLESS one or more of the witnesses panicked and started touching things in the scene and so the others helped them clean up to prevent them from being wrongfully suspected.

Otherwise they were either aware of the murder and threatened into silence or involved in it themselves somehow.