For items 1 & 2:
=IF(ISBLANK(C6),A6*B6*E6,A6*B6*C6*E6)
i dont know if i understand part 3... are you saying that if d6 have a value, you just want to use those 2 cells to calculate a result?
this? =IF(NOT(ISBLANK(D6)),E6*D6,IF(ISBLANK(C6),A6*B6*E6,A6*B6*C6*E6))