Regarding the Level I Tango Intensives, does the letter signify the difficulty of the class? For instance, is Series E harder than Series A?

No.  Each series has its own technique, completely independent of each other.  The classes are not progressive, so you can start taking the Level I Tango Intensives from any session.  The only prerequisite is that you have taken the Absolute Beginner Tango Intensive at least once.