The mapper does not allow access to the destination nodes. This is a restriction of Xslt, not a Biztalk specific restriction.
I am not sure I follow your logic
else If X1=(‘RM’ && !(X1==”KT” || X1==”MT”))
you appear to anding a boolean with a string, should this read
if ((X1 == ‘RM) && !(X1==”KT” || X1==”MT”))