If formula displaying false

Hello-
I am having two problems with a formula. The first being that when the first if condition is not met, it is displaying false. The formula should be set up with multiple if statements and then display 0 if all conditions are false. What am I missing? Why won't it work for all the conditions or display a 0?

Second problem is that when I try to add the last if formula to accommodate the drop down option of 60 months (5) excel is giving me an error in the formula. It doesn't matter if I type in the if statement or cut and paste. Why won't it work for this option.

A sample spreadsheet is attached. The formula is cell F6 highlighted in yellow.
Thanks in advance!