Hi, thank you for the suggestions !
1) Yes, it should be better
2) It was because of an issue of bones relations when used in the deform tool (polygon deformation) but the system could be improved to allow that.
3) Maybe I didn't understand well the question. Several bones can be used on one image in the deform tool :
More detailed process here : https://pixelover.io/docs/tutorials/mesh_deformation/