Can modeling chocolate be used in long term displays? I am working on a gingerbread display that needs to hold up for a month. Is it ok to use modeling chocolate for some of the sculpted elements? Thanks

Most definitely!  I have modeling chocolate pieces that are over three years old and they still look fine.  After a couple of days, a hard skin forms on the chocolate and makes it very sturdy.  Be sure to keep it away from  heat and direct sunlight....it is chocolate and will melt.