There is no need for a modifier for smoothing (starting with version 2.46?). In the mesh tab, select auto smooth and set the angle to 180 degrees. This will cause your sharp edges to show up and export properly without the need to actually split vertices with the edge split modifier.
As far as I know, it's not possible in Blender to have linked modifiers (although someone says you can use library overrides, but to be honest I don't know how to use them). When you create duplicated link you only link object data (that is a mesh). So you need to copy modifiers to linked duplicate manually. More on how…
I'm not sure if I understand correctly what you want to achieve. The weight of each vertex controls the strength of the modifier, if you set the weight to 10%, it will only be influences by 10%. I don't think there is a way to set the vertex weight procedurally, instead you will have to assign or paint them. Edit: You can…
Interesting to hear about the presentation and visual use of the auto smooth option. I might play with that in the future. As for the edge split modifier. The edges get only split if you apply that modifier. Otherwise it's non-destructible like all of Blender's modifiers. And in a game engine, the verts get split anyways…
They still haven't fixed the freaking horizontal panel bug where if your panel is pre-2.5 style (like when you get when you open an old file), then go to the modifiers tab then add a modifier then the fonts and scaling all go to hell.
@justo Yes, sharp, don't know why i said hard xD. Edit: I usually work with the subdiv modifier with the ''edit mode'' option turned off, if you have the bevel modifier the mesh will not look too bad as it holds the shading a bit.