Minecraft: Are light sources additive?

Solution 1:

No, they don't.

Minecraft does indeed simply take the highest of the neighbouring voxels and subtract one to calculate the lighting level (unless it's a source of light, than that blocks light level is simply the light sources light level).

Both ice and water reduce the light level by 3 total each step (1 for the normal decrease and 2 extra). Some technically transparent blocks (transparent because they're not full blocks as far as rendering is concerned) completely block light, namely slabs, stairs and farmland.

Cobwebs and leaves stop the special "light source" that is the sun from going further down, meaning that from that point on normal rules apply to the sunlight.