I am at the last 2 steps of the quest as stated in the Walkthrough Part 3:

- ask Gustapho for permission to arrest Commendoni
- ask both Timore and Servertinus for permission to arrest Commendoni, but they each give you a quest to exchange for their permission

I DO have permission to arrest Commendoni.

BUT I cannot talk to Timore nor Servertinus about Commendoni. They end up 'not talking about it' no matter what conversation thread I use.

Think I MAY have competed ' a quest to exchange for their permission' much earlier in the game when I talked to them BEFORE I used the walkthrough.

What quests do Timore and Timore and Servertinus give us?

I have exactly the same problem

Giovanni Timore will give you a quest called "Remove the curse from the daughter of Giovanni Timore" and Cardinal Severtinus will give you a quest called "Find the statue of the pagan god Bhelfelix". To move on, you have to threaten them both that you will make their secrets public. I don't think you can receive these quests early.

Is there a way to see completed quests?

I think I remember the "Remove the curse..." quest being done. I do not remember the Bhelfelix quest.

By "threaten" did you mean in conversation thread? I did that, and still eventually get to where they refuse to say 'anything about that.'

No, I don't think you can see completed quests. You can receive this quest only when you threaten Timore with this sentence: "I'm sorry, Your Excellency, but I'm not going to leave till I'll get your approval. I'm sure you'll give it to me in the end. Surely you don't want everybody to get to know about your secret..."

For Severtinus, the right sentence is: "I wouldn't do that if I were you, Your Excellency. Definitely not, unless you want the truth about yourself to come to light." I dug deeper into the script and it seems that Cardinal Letti must advise you to threaten them for you to be able to do that. Check your Journal - if you don't have a note in it saying: "It is doubtless that Cardinals Severtinus and Giovanni Timore were corrupted by the Children of the New Order. I must pressure them to grant me their consent to charge and arrest Inquisitor General Commendoni at any cost!", you should go back to Letti to discuss the situation.