Cycles Diffuse Bake Failed :(

The problem lies in the fact that the low poly version overlaps with the high poly. You need to desable rendering for the high poly one ( the camera icon next to cursor and eye icons)


as for blender render, I think yes, you probably need to re-do the material (but there is no need to bake a diffuse with blender render)

Did you duplicate the geometry to create an inner side? Do not do this, we need it only in places where it is visible. And as you create new geometry, always move its uvs away from the original

Do you have any body parts under the rig? Disable rendering for them as well

if no, share the blend file please

I cannot quite recreate the situation because the textures in nodes were not preserved, hdri and the normal maps are blank,, but i do not get the black patches, i think you might be baking things wrong. You are also making things unnecessary complicated for yourself for the first try. Remove hdri and all materials for a second

- select all uvs and assign a new blank map;
- create a new image node, choose said image. Do not connect this node to anything. This image node would need to be present in all materials;
- optionally reduce the samples and light paths. You can always set light paths to 1 or 2
- bake



Also, the mesh is vertex painted with the wrong color, should be 00ff00. The uvs should be transferred once the mesh is fully modeled, not in the process. Right now there is only one half of them. Also, make sure to delete those custom vertex groups for the belt and the rest

To have transparent background you need to create a texture with transparency. So when you click on new, there will be the background color option, click on it and you will see A field at the bottom (it controls the opacity of the background). Set this thing to 0.

It looks pixelated because the samples are small. I usually bake the textures with 2000 samples but it may require more with hdri. It takes more than an hour to bake this texture for one single shoe ( my pc is pretty weak, though)
Before you bake the final result, make sure you are 100% satisfied with the lightening, material properties and so on. Check all of this on low samples.
How to test how many samples you need:
- save your blend;
- select a small area under armpits or anywhere else where there are prominent shadows (shadows is where the most pixelation occurs), delete everything but that area;
- start with 500 samples. Increase to 1000. If the noise still exist, increase to 1500, then 2000. If, for example, the noise will disappear at 2000, try baking with 1800 to see if this is enough;
- once the bake it clean, close the blend without saving.

There is a denoising tool in the newer versions of blender that will allow you to bake something on low samples and remove the noise after but it may remove fine details. You can look up the subject and try it, but if you have a few hours to spare, it is best to bake it on high samples


to answer the questions:

1). if you are baking diffuse or combined, yes, the light will influence the result. If you want to bake just the color (plugged in texture) information, you can do this by baking Diffuse color. Otherwise, no, the lights will come into play. Remember that without the light there can be no shadow.

2). As I said, 3fff00 is meant for skirts and in some cases breasts, it disables certain morphs (well it does not disable them, but it will make the mesh behave as if those morphs do not exist). For example, when you will paint the long skirt with 3fff00, the bottom of it will stop being affected by leg morphs. If you change the thickness of calves, the skirt should not change, right? 
If you want to see the difference for yourself, you can clone a maxis dress with a puffy skirt, export the blend, paint the whole thing with 00ff00, reimport, and check it in cas. Like, check how the edited version morphs and animates as compared to the original.

3). yeah, I assumed you transferred uvs when the mesh had the mirror modifier, then applied it.

It’s not a big deal but choosing references with as little sculpted details as possible will minimize the amount of rips.

Switch shading to Solid and look at the mesh in those areas.