That is a lot of topics for one question! The white bloom may appear more often on chocolate of poor quality, but even good chocolate can develop a bloom when it has been exposed to too much oxygen, moisture, or fluctuating temperatures.
Preventing the bloom is largely a matter of proper preparation and care of the chocolate, whether made of the best ingredients or not. Most of the bloom results from separation of ingredients such as fats or sugar crystals onto the surface, either by diffusion or crystallisation. The nature of the bloom is then affected by the texture of the solid ingredients. The bloom is hardly important in cooking chocolate, but it does spoil the delectable smooth surface of a high quality chocolate slab.
The causes of pitting are varied, it can result from bubbles, the effect of temperature changes on powdery ingredients, crystallisation, unfortunate choice or preparation of powdery components, and so on.
Rancidity is generally the result of oxidation of certain unsaturated fats. It suggests too much exposure to air, and unsuitable choice of ingredients. Pure cocoa butter should not go rancid, but of course it is expensive. Mind you, don't be too uncharitable; nuts in chocolate are the most frequent source of rancidity.
The apparent drying out of caramel is not mainly loss of water as a rule, but crystallisation of sugar, turning the toffee-like texture into a powdery or gritty texture. This is not normally regarded as a quality problem unless the confectionery is definitely stale. The fact is that many people enjoy the resulting "short" texture for which the late lamented Callard and Bowser's nougat was famous <siiigh!!!>. Certainly the crystallisation reduces the tendency to remove tooth fillings. It also reduces the subjective sweetness of caramel, much as crystallised honey tastes less sweet than liquid honey.
Cooling chocolate hardens it partly by solidifying some of its liquid components, and partly by causing the molecules in the solid components to pack more closely and stiffly together. It is much like the hardening effect of cooling butter.
Food chemistry and physics is an absorbing topic, but complex.
Bon appetit,
Jon
