Completing German master's programs in less than allotted time
This is something that you have to ask the respective offices at the schools you apply.

You can savely assume that completing a Master's in Germany in less than two years is in general possible if you complete all required things in time, i.e. you've got enough credits in all required categories. It will not hurt if you double check with the "Studien-" or "Prüfungsordnung" of the respective programs.

If the program will award credits for courses that you have taken somewhere else is something that nobody but the program office can answer (and will probably finally decided on an individual basis by the dean of student affairs).