Problems with texturing, and with the surface subdiv levels

[Sub-Zero]

1. How to select a part of the surface of arbitrary shape, and copy the texture to another location? "Transform / Copy" copies only the rectangular pieces.

2. I upload photos to Materials and want to crop a portion of a photo (leaving an uneven piece). When I was in the mode of "Paint over image", I erase using Eraser, it becomes white, but is not removed entirely.

I paint the character, using several photos with different lightning. Are there any advices to produce seamless transitions between them?

How does the "Copy and paste parts of surface" work? It throws a window with "save" and "load" buttons and I can't draw anything.

In the Surface Mode. Is there a function, such as Subdivision Levels in ZBrush? I want to paint small parts at a high level of detail, then I return to the previous level to make a major change (moving the body part of the character), then to return to a high level and continue with fine details,... back and forth

[Sub-Zero]

5 - solved (?), I found an option "Downgrade the volume" in vox layer panel. But it's like there are only two levels: low (proxy) and high (mesh itself).

[Tony Nemo]

Use "Resample" to lower the res (by less from the 1 default, like ,5 for instance will reduce 50%). The 
Live Clay" tools will allow fine detail by increasing the res locally.

[TimmyZDesign]

Also for number 5 (in Voxels menu):