Yes, in general. A category and a subcategory are practically the same in terms of why we would add one to the site. In one specific instance (adding a top-level category), it is much more difficult to get one added, so adding a subcategory is definitely easier in that case.
No.
Simply because 'science' is an academic subject - while 'scientist' is an employment position. Yes, I (and probably many others) agree they belong together because they cover the same topic - but they are separate subjects.
hallucinogens and the dissociative are in the same category because
yes it is in the same category.
No, but there is a Blogger category, which is the same company.
They are all together because they are all one generation of Pokemon. Even though Emerald came out a little later, it featured the same story. There is no need to make 2 categories for something that makes perfect sense in one category.
Yes. They are the same.
The rules are the same.
no it is not
There can actually be a multiple number of supervisors in one category, I've once seen 4 supervisors in one category! But it depends on how busy the category is, sometimes and usually the category may need more then just 1 supervisor for one category of interest. So there are usually a multiple number of supervisors, supervising a category. But usually less than 5, but hey, the more the merrier!
Sadly, no, you cannot. Another Supervisor might want to supervise that same category, so no 'rain checks' are allowed for categories. However, when you are old enough, you can co-supervise a category with another category supervisor for that same category you wanted to supervise.
the shaker rules are that they all have to be the same