The 1903 Safety blocks the Sear when engaged. It blocks the Sear from movement right opposite from the Sear's engagent with the Hammer.
The Grip Safety also blocks the Sear. When the Grip Safety is extended out (On Safe) the hooks on the top front of the Grip Safety sit atop the matching platform curving upwards on the Sear.
Closely hand fitted, the Grip Safety will ensure no movement of the Sear when the trigger is pressed.
Disengage the GripSafety, the ThumbSafety still engaged will prevent the Sear from movement when the trigger is pressed.
Again,,this all depends on close hand fitting.
Pistols that have had parts swapped out and exchanged, clumsy gunsmithing done to them, parts simply exchanged and left w/o further examination because the mechanism 'Seems to work OK' may have serious safety problems.
The Hammer does have a half cock notch. It's there as another safety devise though the hammer being completely hidden.
The half cock notch is there to catch the hammer should it fall from the full cock notch and to catch and hold it there so the pistol does not discharge accidentaly.
I've seen these where the 1/2cock has been removed . Disconnectors altered, grip safetys pinned in the 'Safe-Off' condition, Full Cock sear notches worked over for what I can guess is a better trigger pull.....all that can often lead to an accidental discharge and possibly someone getting shot.
As far as the firing pin is concerned. It's a 2 piece FP. The front half is spring loaded to retract.
If a FP lock was needed, I think there would have been one on it at the Militarys demand late in production, much like the S&W M&P change to in hammer block.
The 1911 didn't have one originally. In the 1930's with little to do, a guy at Colt decided the 1911 might need a firing Pin Lock.
So we had the Swartz Firing Pin Lock System for a while on Commercial Colt 1911.
The Military never went for the trinket AFAIK.
The '80 version nothing really new.
Re: the Shooting Death,,,,
If the self inflicted wound from the dropped 1903 happened as they state with the pistol carried w/a round in the chamber,,,,and I assume the thumb safety engaged ,,,,
IF it fired when striking the floor in that condition,,,
The pistol should have been found with the fired casing still in the chamber of the pistol,,
The thumb-safety still engaged,,
Internal hammer still cocked
Then I'd believe that the firingpin inertia alone fired the cartridge in the chamber...