I'll have a crack at the first two;
1. The wall thickness needs to be the same at least to past the lowest point of the natural edge. The reason for this is if it's allowed to widen the lowest bark/natural edge will be thicker - difficult to explain - look at the pic's the bark area gets wider.
2. This is more a personal thought - the walls should not be to upright, curve them under the form faster, get away from the upright walls of a bowl. It makes the form more pleasing to the eye.
Eh... nice carving of a thumb but don't think it fits the look of the bowl!