Why might the German ambassador have used such an elaborate code to send his message to Mexico?

to avoid it being easily picked apart by interceptors, while yes it could be decoded they literally had no other way

the protestant reformation's started in 1517, disturbing the religious and political order in europe´s catholic countries. the outcome was the european wars of religion prosecuted in europe in the 16th, 17th and part of the 18th century. nonetheless, uprisings, territorial disputes, and great power conflicts were also causes of the wars.  

d. promised it other land