When you retouch a texture, If I understand you are referring to editing a flattened 3d texture map (of a figure) hopint to reapply it to to the figure. It can be done if you just overright the existing image name, however I would not recommend that. Its better to switch the referenced file-name in the figure. Either way automation adds little since its uni directional (injecting a new map).
With respect to the 4 bullets, Other than the first, these can not possibly be accomplished in GIMP since it does not posses the capability to import the 3d scene objects, no render them. You could make it so that DAZ exports scenes as image layers - but for what purpose. If scenes had multiple layers in DAZ like they do in E-ON Vue it could offer some benefit. Or if your talking about exporting the alpha, depth and color channels as independent layers…
- Launch DAZ Studio from Gimp and vice versa.
- View 3D scenes as Gimp layers
- Render Directly into Gimp (HOW ???)
- Import and export image maps onto 3D models in Gimp
I guess I just do not see GIMP taking the place of either PS nor ZB. Have you checked out “RealityPaint 3d” - its quite affordable and does much of what I THINK you want to accomplish, easily. Especially helpful is the PER-PIXEL mesh painting capability where you paint in 3d on the surface without worrying about mesh density and the mesh does not even need to me equalized to get good results. See realitypaint.com