The spy mission will not give you that last cross im afraid.
Spy style missions only ever give you renown points (spend on your boat upgrades and they count towards your campaign points).
So that missing cross is because you missed an objective worth 1 cross (could be that you skipped the tutorial
). So now your campaign will end when you get to the end date or finish the last objective rather than getting all the crosses. This will not effect your career and you will continue as normal with a good victory
You could however edit your save game to tell it you have that extra cross
Edit: if you install OHII - you will get more mission like the spy one