#1 happened often.
Soldiers were drilled to load, aim and fire in a fixed rhythm to maintain volley fire. In a well-drilled unit, this was muscle memory. If a soldier's musket did not fire, they may not notice with all the other guns going off simultaneously. The order to load would come, and they would dutifully pour more powder in and ram another ball down. The result would be something like this...
> spark source = powder | wad * ball - barrel
When fired, the spark from the primer would fall on the same inert gunpowder as before. The new powder would be blocked by the ball and wad. Click.
As volley fire continues, there will be more smoke and more very, very loud noises going off in the soldier's ear further befuddling them, but the drill will continue. Load, aim, fire, click. Load, aim, fire, click. Load, aim, fire, click.
At some point they'll notice they can't ram the ball down nearly as far as they should, but they may also think the barrel has been fouled by black powder residue and simply ram harder.
This scene from Glory shows an officer simulating battle conditions to rattle a soldier and impress upon his men the importance of drill.
#2 is covered by User58220 and sounds plausible to me.
#3 does not make sense, it would have to happen 20 times.
#4 is a very problematic way to store balls. They could get stuck in many, many ways. Balls were generally slightly smaller than the barrel, but this was in no way guaranteed. Lead is soft and manufacturing techniques were not very good. The balls could easily be misshapen in manufacturing or handling. Barrels became fouled by black powder residue from firing and the balls may no longer fit so loosely. While traveling, jarring the gun might cause a ball to jam in the barrel.
To ready your gun, you would first have to remove all the balls from the barrel and be sure you got them all out. There might be days between loading the barrel and readying it to fire. "Now did I put 18 balls in there or 19?" If a single ball gets stuck, you cannot fire the weapon and must disassemble it for cleaning.
Then you have to put them all those extra balls somewhere while you fire and march. If you put them on the ground, you'll lose them or march away from them. You need a bag or pocket, which is where you would have carried them in the first place.
Furthermore, militaries would not be carrying ball and powder separately. They used paper cartridges to increase their rate of fire. The powder and ball would be wrapped in a convenient package. The paper would be torn, powder poured down the barrel, then the wrapper for wadding, and finally the ball. Here is an excellent video showing the making and use of paper cartridges in the American Revolutionary War era.
Tubular magazines do exist, but they came much later and for breech loading repeating rifles.
In contrast, a bag has none of these problems.
Finally, muskets, cannon and pistols were sometimes "double-shotted" by loading two balls and extra powder, or more often a ball and some buckshot. Loading this way was a slow process, and put extra strain on the barrel. It was typically only done for the first shot and at very close range. This would only work with a smoothbore weapon and not with a rifle.