You should be under no obligation to bail them out if they failed. Have them get picked back up, and asked to report about what they found. 
 
"We didn't find anything, just got the tithes back up!"
 
... So what was the purpose of the Cardinal's letter?
 
"Uh, I'm not sure...."
 
They failed the mission, so punish them for it. Loss of Influence, berated by Interrogator, etc. Then get give them a boring mission like having to go get the Inquisitor's laundry, where they witness people eating food and spontaneously mutating. They kill them, look at the food, and find it is from Novabella...

Talked with the group. I think they know they are not finished yet. But they thought getting the tithe rolling again was the most important thing. So I think I will play out the rest and maybe they will have an "OH F$CK WE SHOULD NOT HAVE DONE THAT!" and then let the tithe thing come back later.

So just had a session and testet out some ideas. And I have to say it worked.
 
Thank you guys. You are as always great.