1) The "mean" filter is more a way to find the most frequent value. If you have 5 layers, on a pixel position where two layers are black and three are white, the result is white, while the average would be some gray. It is typically used to find common pixels among layers. For example, take several shots of the same monument with tourists walking by, the median filter will produce an unmasked monument, while the average filter will just blur the tourists. On the other hand the average filter is appropriate to remove noise, if you consider than noise is a random difference with the "true" value, centered around that "true" value.
2) That rule is mathematically sound and there is no other approach (except an average filter). In practice with Gimp 2.8 you get round-off errors and you cannot apply that rule to more than 5-6 layers (above that dark values are rounded off to 0 and don't appear in the result), and the average filter is a better solution.
2) That rule is mathematically sound and there is no other approach (except an average filter). In practice with Gimp 2.8 you get round-off errors and you cannot apply that rule to more than 5-6 layers (above that dark values are rounded off to 0 and don't appear in the result), and the average filter is a better solution.