Strength and skill can't be developed through methods that employ constant variation.
Correct, not until the skill of an exercise is achieved, strength can then progress
Strength is the most important physical attribute, improving all other attributes, like speed, agility, balance, and power. Basic barbell training is the best way to build strength.
Basically correct in terms of performing exercise, which are all by-products of strength and most important..flexibility.
Basic barbell movements are the best way in terms of simplicity but does have limitations in terms of proper exercise.
CrossFit and "functional training" completely miss the point of basic strength training.
Most methods do IMO
A wobble board squat can't translate to a skilled performance in an actual sport, unless the sport is wobble-board squatting with light weights.
Correct
An increase in strength always improves athletic performance.
Mostly, to a degree.
Muscles have the ability to perform unlimited movement, to a point where nobody really understands the functions, if one says they do they are a fool, Nerves that originate from a given muscle for example cross over other muscles and in some circumstances just end randomly.
So it would then also makes sense that a limited (small) number of "proper" exercise is all one needs to benefit.