This quote from Cecil Gray's (now old) book The Forty-Eight Preludes and Fugues of J.S. Bach London, Oxford University Press, 1938, may help underscore the fact that not every fugue by Bach is a text-book example of fugue writing.
"A few words concerning the nature of fugue may perhaps not be amiss here. Fugue is, practically speaking, the only tolerably strict musical form that there is. [...] At the same time it is not as strict a form as, say, a sonnet is, consisting as it does of a fixed number of accents in a fixed number of lines, and demanding a fixed scheme of rhymes. [...] there is always a certain amount of latitude in the procedure that can be followed. The amount of latitude that Bach permits himself, incidentally, may be gauged by the fact that none of the recognized authorities who have written treatises on the subject such as Fux, Albrechtsberger, or Cherubini - ever even mentions him. In the form as Bach practised it only one principle is invariable - that of the exposition so-called, in which the subject is first announced alone, and then answered by another voice, while the first voice continues with a counterpoint to it. This counterpoint, however, may or may not be a regular counter-subject, as it is termed, which recurs throughout the fugue. (In this connexion it may be mentioned that in eighteen out of the forty-eight there is no regular counter-subject.) Again, the nature of the answer mentioned above may vary; it may either be what is termed 'real' or it may be 'tonal'. In the former the subject is repeated, note for note, in another key - almost invariably the dominant, or fifth above; in the latter the subject is altered in such a way that where the tonic originally appears in the subject, the dominant appears in the answer, and vice versa, and with some subjects this may somtimes involve the alteration of other subsidiary notes of the subject."
Does this help to answer your first question?
Regards,
