It is actually not necessary, but quiet usful for a good understanding of general chemistry (with regard to the first semester).
Of course, it is possible to change a course of study. However, it is not possible by enrolling in pharmacy studies e. g. e. g. to “pass” the NC or to get a place in general easier. You still have to apply regularly via university start. We would also like to mention here that pharmacy is not an alternative to medicine. Those who actually aspire to a medical degree will not be happy with the chemistry-burdensome pharmacy. Furthermore, several completed courses can be credited, but not entire semesters.
This is so general that it is hard to answer. What is clear ist that it is a very time-consuming (a normal day starts at 8.30 a.m. and ends at 6 p.m, plus writing minutes, preparation and follow-up, learning for exams and colloquia...). Whether the course is accepted as difficult depends heavily on personal understanding of the natural sciences and other organizational knowledge. Nevertheless, if that's what you want to study, studying is really fun :)
First of all, we recommend that you contact the representatives of the Examination Commission from our study council.
They can advise you and explain how to proceed in such a case.