Stuck on correct function(s) & formula!

On my spreadsheet tab 'Scores and Assigned Priorities', I have a Baseline score (column D) and a Next Assessment score (column E).

I need column G to:

1. know the difference if column D has a score and column E is empty
2. rate the score in column D when it's by itself AND (E minus D) only when both have a score
3. rate the D or (E-D) score as 1=high, 2=medium, 3=low, 4= NA, NA=NA

This is what I have so far, but it's not working when D is the only column with a score in it. I've tried a bunch of OR, AND functions, but that didn't help. I'm out of my league :D

=IF(SUM(E71-D71)<=0,"HIGH",IF(SUM(E71-D71)<=1,"MEDIUM",IF(SUM(E71-D71)=2,"LOW",IF(SUM(E71-D71)>=3,"NA"))))

Thanks for any assistance. The file is attached.
Catherine