Is Column A a general categorization of each record indicating how long is allotted for completion? i.e. Critical gets until Start + 15, Severe gets Start + 30? Is that right?

If so, that doesn't seem so complex. However, I am not clear on your "bigger monkey wrench". You indicate wen you want something to happen (i.e. when not past due), but not what you want to do. Do you want these to remain blank or something else?

I've worked one up but I can't seem to attach anything right now. But basically, what I did was:

Make a table (I named it "Levels") on a separate tab for the lookup values for the levels
Level Due
Critical 15
Severe 30
High 45

For the main table, I have three columns: Level, Start, Days Past. The equation for Days past in Row B is:

=IF(TODAY()-B2-VLOOKUP(A2,Level,2,FALSE)>0,TODAY()-B2-VLOOKUP(A2,Level,2,FALSE),"")