Wow there are alot of questions since I last posted, dont have time to examine all of them so here is my workflow, maybe some if not all your questions will get a answer.
1: Design. If I have a set design and have images front, side, and some action poses, then Id model it within C4D pur polygon modelling getting all the edge flow in place. Assuming this is for animation id take care to have loops for all articulation points such as shoulders, elbow, knees e.c.t.
I would model the eyes and hair, teeth, and all accessories separate, and if the character has non dynamic clothes the clothed areas would replace the body under it. In that case id model what is to be seen only such as hands, feet. If its to have dynamic clothes then I model the body very basic, do some sculpting to get the base shape down then build clothes around that in MD.
2: Zbrush base level: Assuming I had made a good topolological model in c4d Zbrush would be used to refine the final shape, and then id sent it back to C4D. If The model was designed within Zbrush and has bad topology and high dense mesh then Id send the base level sculpt no detail just volume over into C4D, scale it as it needs to be for the world space I need it for, then send bring it into Zbrush as a new name. The matching of names will cause the previous version to update in Zbrush and this can mess the scale up of the model and the brush size, so to avoid that I import the base version scaled correct from C4D into Zbrush as a new model. Zbrush will see this as a new mesh and apply the scale difference to what it needs in meters with the correct export setting for C4D. Id them start to sculpt.
2: Zbrush sculpting: If its got its final good topology done from the beginning id store the morph target, sub divide the mesh and add detail using the layers. id keep larger deformations in one layer and small details in another layer this way I can capture only micro details into the normal map, and large volume changes into the displacement, however I do my best to make the topology hold its own volume.
In the case that the mesh is made in Zbrush has bad topo but is the correct scale for the game engine id not bother with store morph target as the level is still a work in progress but its got the right scale. Id work on volume for a while, then only add the sub d levels as and when I need to, this avoids lumpy uneven surfaces. I wont add high detail to this version, only main forms, no skin details. Id send this over to C4D and use the polygon pen to retopo it. The re topo in C4D is faster than within Zbrush. Modo is even better.
3: The retopology will be given priority to jointed areas for animation, secondary is topo for anatomy, and I will retopo manual the areas that are going to be rigged and animated such as body. Zbrush remesher for some cases can be used such as hair, if this is going to be a static moving object with the head then Zremehser is fine. If its going to be animated such as long hair I would have modelled this correct in the first place and do away with any proxy hair made in Zbrush that aided the design to start with.
With Topo done, the hands, feet will normally be modelled as well as hair teeth and accessories, retopo can take long for some things in which modelling is faster.
Id send over the retopo version back into Zbrush as an appended object into the sub tool of the high res mesh, then project the lowest level of the low level Zbrush version into the low level of the retopo version. Id move up one level at a time and project each level. If the retopo model is good to the point it holds all its own volume id store morph target as to retain the base cage mesh that matches what is already in C4D, but its not always the case id do this as the projection helps push them forms into cage a little more.
When I have the projected version I d clean up scene deleting the high res version, id append in the teeth, hair, and accessories into Zbrush and save this as the final version. Id export a obj cage mesh for all parts and do my Uv work in Unfold3D. With the new Uv version id import these one at a time back into zbrush which will update each sub tool with new Uvs. Id save the Ztool again.
4: All parts will also be taken into C4D ready for rigging. In the Zbrush version Id finish the details and painting.
The details will be stored in its own layer and if any volume has been added that will be for displacment, or pushed into the cage mesh if no displacements are wanted. Should I have already rigged the model and later found that I had a better cage mesh produced in Zbrush id use pose morphs to update the rigged model.
In Zbrush Id go to high sud d level and paint the parts. Id produce the maps without smooth Uvs and I work in linear Uvs. Id take the maps and enhance them in photoshop. I may use substance painter for some parts like clothes, but skin will be done in Zbrush and Photoshop.
5: All maps are brought into C4D applied then the surfaces will be made refined later on. Id rig the character if Im used to making the correct topology id be at this point, but If I was learning it id not make any maps or Uvs, and test my deformation for rigging as I may need to add or re do some topo and you may end up messing up the Uvs, and the maps are useless.
You got to find a workflow that suits you, and be open to improvise and adapt to new problems as they arrive.