So there are theoretically descriptions that could be written for the Dark Side of the INFP, while still being just as INFP as the Bright Side. No MBTI letters or type functions are inherently happy or good-natured.
On MBTI vs. Type functions: They are kind of two separate theories, even though based on the same ideas, so it is possible for you to "accept" one theory while rejecting the other theory. Many people prefer one over the other; others prefer a reconciled combination of the two.