Thank you hihosilver, you're too kind, really. Maybe I look like the smart guy who plays dumb, but I can assure you that this is just the good luck of a starter - and by the way, I used a reference image, of course, along from stealing ideas here and there from the tutorials and from all of your works people.
Here are the rigging and the skinning problems, my internet connection comes and goes, I'm attaching the pictures and editing this post to make the short story long...
...so here is the edit:
Skinning image:
Left image, the oval on the first joint of the thumb shows where it still bends wrong, and the other highlight shows where the skin makes a weird line when the thumb closes in.
Right image: the top side of the fingers curves while there should be a straight line following the bones.
Also in the upper palm, there should be a bend where marked by the red circle. This is where I'm planning to add bones to keep the volume.
Rigging image:
So far the two bones added to give volume to the thumb's base are working fine.
The other two bones highlighted in the rigging image show how I rotated them to help the hand to close better, moreover to avoid the back from staying all on a plane, when the fist is completely closed.
Looking forward for your advice, thank you very much for your help people.