It's not possible because comparing two fields against each other requires a row-level evaluation but set analysis is a column-level evaluation. Further NULL doesn't exists and could be therefore not directly queried within a set analysis.
I suggest to consider to transfer the essential parts into the data-model which might be just:
coalesce(sign([lease end date] - Final_Term_Date), 2) as Flag
and then something like:
count({< Flag = {0, 2}, [continuing occupancy] = {'N'} >} [Lease ref])
