The hammer spring does play a role in preventing pressure from spurting back through the nipple channel.
Too many shooters forget that when gunpowder (old or modern) is ignited, the pressure it builds initially pushes in all directions. Firearms work on the principle of "seeking the path of least resistance."
Firearms are designed for bullets to be that path of least resistance. When the bullet IS NOT that path, mud in the barrel for example, the pressure seeks an alternate route.
With cap and ball revolvers, that path is back through the nipple channel.
When a cap and ball revolver is overloaded, such as using more Hodgdon 777 than recommended by the manufacturer, pressure may be high enough through the nipple channel that the hammer is brought back to half cock.
In the first half of the 20th century, there were numerous smokeless powders made to mimic the same bulk as black powder, for use in shotgun shells. This made reloading easier, since the same measure for shot is also used with black powder. Almost immediately, old black powder rifles and handguns were damaged or blown to pieces by the use of this bulk smokeless powder. Pressures were just too high.
In my collection of old American Rifleman magazines (1929 to present, an unbroken set) there are numerous warnings against using bulk smokeless shotgun powder in anything but shotgun shells.
And more than one photo is shown of the catastrophic results of straying from that warning.
I recall one note that a Colt cap and ball revolver was so heavily overloaded with this powder that the hammer was brought back to full cock, by the pressure spurting through the nipple channel.
Interestingly, the Browning Hi-Power 9mm pistol typically has a 32-pound spring for its hammer. This very heavy hammer pull is on purpose; the strong spring pressure retards the slide from coming back too fast and battering the frame. Basically, the same principle.
From what I've been able to determine, it's a myth that extra cylinders were carried. I've yet to see anyone cite an actual source for this: diary, combat report, newspaperman's observation, whatever. Nor has anyone discovered an old, contemporary holster with extra pockets for loaded cylinders. So, lacking any documentation, I don't believe it.
I've never liked a wedge that could be pushed out by fingers. It's just too loose. I suspect it leads to a battered or widened wedge slot in the arbor. I've found best accuracy with a fairly tight wedge. When the wedge is loose, my groups widen.
As Mark Twain observed, "A difference of opinion is what makes a horse race."
"A vast desert. Galena in flight. Smoke. Brimstone. Holes in parchment. The ugly cat is much amused."
-- The Quantrains of Gatodamus (1503-1566)