The grainy texture is likely to be a bumpmap detail that the game applies on ped* shaders. These details are on "ped_detail_atlas" texture located in common.rpf/data. and the sample from the given atlas texture is selected according to blue value of your specular texture.
I've just made a post on this issue here:
viewtopic.php?f=31&t=11975&p=59756#p59756
Concerning the detached head and hand models, you need to paint weights perfectly on a seam. I don't have a good solution for this issue, but if this could be an option, you can do this as follows:
1. attach hands/head to torso. (this could be the problematic point, as the final mesh can not exceed 64K of vertices). If this is a problematic point, create a copy of torso model first, then delete most of torso polygons, except the very tips (belt of polygons) that are neighbor to hands and to head, the rest is of no interest and you can delete it. Then attach head/hands to this remaining "pieces" of torso.
2. switch new object to vertices level. Use Select\Open edges tool to select vertices on hands and on head that are neighbor to those remaining pieces of torso. In short - you need to select vertices on a seam where you get a problem in game, but you need to select vertices of head and hand (not vertices of ex-torso geometry).
3. use rigging\influence\copy tool on selected vertices (pick a tool and click on any of selected vertices. the tool will copy weights from non-selected vertices on matching positions - it will read weight from ex-torso geometry.
now you have an identical weight on vertices of head/hand on a seam matching weight on respective vertices of torso.
4. delete fragments that belong to ex-torso. you can use Select\Separated tool to select them quickly on vertices or polygons level.