> Quote - ... I don't think you can create a shape key additive to another one (that is, with a previous one applied and visible in edit mode), but I'm willing to be corrected on that point.

You can enable shapekeys in edit mode by clicking on the small cube-lookalike button (see illustration). That enables the shapekeys in edit mode according to their current value. That includes the shapekey you are currently modifying, though. So to use it, you have to dial the shapekey to 1 before going into edit mode, otherwise you will not see what you are doing.