Simply put, "It has value" is qualitative, as in "valuable".

"It has a value" is NOT quantitative, it's simpler than that: "It has a value" can only be interpreted as "It has a number [associated with it]".

When saying "I have a..." you are referring to something you are in posession of, whether tangible (I have a shirt) or intangible (I have an idea) that can exist in singular or plural form (shirts, ideas). Then the "a" refers to "one".

In this context, the word control is a concept (i.e.: the ability to control) rather than the noun "control" (e.g.: the mechanical device that controls). As such, you simply say "I have full control".

Why focus on people? Maybe inanimate objects might offer some fresh perspective.

"A few tables were placed in the banquet hall, but few of the guests dared sit at them."

I would tend to say that "a few" refers to "a small number of"... while "few" means "less than expected".

Perhaps it means that it isn't evil, whether observed from within or without. I see an internal vs. external reference in "in and of".

I think it's more simple than all of these explanations.

While it's in original animal form (whether dead, live, boiled, fried, hanging in the window of a chinese barbecue shop or otherwise) it is pluralized. (Look mom, I've got 2 boiled chickens!)

When referring to the meat, it's simply chicken.