Thanks ! For the seams, it's 3D coat baking, i'll try Blender solution. But i made standard UV , it should work directly, But when 3D Coat re import the low poly model it makes the seams.
For the head proportions ? I don't know but ib the real life all people have really really different heads, really. Some have a big mouth , some other a bigger top head, anothers some really large head , some have little heads than body and some it's contrary like bigger head than body etc ... etc ... Perhaps observe people of any age in real life ? I don't see where the head is so wrong ?