It seems like your case can be simplified. You've already verified that Start_DT is null with your first condition, so then coalesce(Start_DT,'1800-01-01') seems like it would always return Start_DT. So your second condition is just checking that Start_DT < '2015-03-01' and Start_DT < '2016-10-25', but if the first part is true, the second part must also be true, so it simplifies further to just Start_DT < '2015-03-01'. Since that's our second condition, we'll only get to our third condition if it's >= '2015-03-01', so there's no need to check that, and the second part again simplifies to Start_DT < '2016-10-25'. So it seems your case statement reduces to this:
Case When Start_DT is Null then Null When Start_DT < '2015-03-01' Then 'Yes' When Start_DT < '2016-10-25' Then 'No' END
Now in QlikView, if Start_DT is null, then if we check that it's less than some value, this will be false, same as checking greater than a value. Null isn't a value. So in QlikView, we don't need to check that first condition. We can go straight to the date checks. I believe this would work.