For number 1 you would just do something like:

=if((SUM(MD!C5,BI!C8,Mny!C5,Theft!C5,EL!TABLE_29,PL!F5))=0,Nil,SUM(MD!C5,BI!C8,Mny!C5,Theft!C5,EL!TABLE_29,PL!F5))