I am thinking of making a world map (a heightmap) and editing it in Wilbur (adding erosion, riverstreams etc.).

If I use a projection like Mercator or Equirectangular, Wilbur definetly won't understand it. The rivers will be the same from Wilbur's point of view, but in fact Wilbur will make real, large rivers only near the equator - the polar "rivers" will be more like little streams in the mud after a summer rain. 5 pixels near the eqator are not equal to 5 pixels near the poles. So I will probably need to use an equal-area projection to avoid that. The distortion of angles will happen, but it is not that problematic... right? Actually I am not sure I am right here...

On the other hand - may be the map should be cut into several pieces for separate editing? The equatorial cyllinder and the polar circles? Then those parts will have to be merged later and the problem of dealing with the stitches rises.
Probably cutting the map into pieces is a better solution because it will allow you to apply different erosion settings in different regions - but, once again, what about the stitches?
It seems the river map is probably going to be terribly distorted... (If you "cut away" the upper part of the river Wilbur will never know it existed.)

The perfect solution would be combining best from both worlds - finding a good projection for rough editing of the global map, and then finding a good solution for the problems that rise when you polish the map region by region.

Has somebody already faced these problems? All the Wilbur tutorials I read do not cover the issue with projections or cutting maps into pieces and sewing them together again (or at least do not concentrate on them).