If statements with <>, blanks, and errors
Need assistance putting all my "if conditions" into a single formula if possible
Answers

First, can you add an IFERROR(*column formula here*,"") to the Due column to make it blank instead of giving an error?
Then use this formula:
=IF(Due@row="",0,IF(Due@row<Sent@row,1,IF(AND(Due@row<TODAY(),Sent@row=""),1,IF(Due@row>=Sent@row,1,IF(AND(Due@row>=TODAY(),Sent@row=""),1,"")))))

=iferror(if(or(due@row<today(),due@row<sent@row),1,0),0)
Seems like it should work. Your records 2 and 4 are just the false conditions of your records 1 and 3, so you get them evaluated for free. Your 1 and 3 apply as an "or" condition (it isn't due yet OR it got done early) so both of your false conditions are an implied "and" (it is due AND it isn't done). You can put whichever one of them you want first, but if you go with the result 0 values first make sure to choose an "and" test vs an "or". Incidentally  with your conditions the way they are, if someone gets it done on the day it is due you are counting it as late.
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