So where you messed up was in post #4 where you asked about the grades "ABC" which is text. In column C you have numbers 123 so the formula in CF cannot be
C6="1" but must be
C6=1 .
If you use double quotation marks it goes without saying that you are dealing with text and as you well know in Excel "1" (text format) is not equal to 1 (decimal format).
I leave you the task of fixing the whole formula

.
PS. forgot about when J6 has a date, does it still need be highlighted if over date ?
If so then the formula in CF could be:
=AND(J6="",OR(AND(C6=1,TODAY()-I6>30),AND(C6=2,TODAY()-I6>15),AND(C6=3,TODAY()-I6>7)))