So are these characters dead or not ?

I know that when it comes to Doctor Who, there is usually no clear answer as to the fate of certain characters.

However three in particular have been bugging me since Twice upon a time. 
Clara, Bill and Nardole. 

Clara died in Face the Raven but was extracted from her death in Hell Bent two episodes later. From what I understand she always dies on trap street regardless. All that the events of hell bent mean is that she has adventures with Arya but eventually Clara has to go back and die anyway. Is this correct?

As for Bill and Nardole, we learn that Testimony take the memories of someone who is nearing their death, so does that mean when Bill became a cyberman she was actually dead for good ? And heather bringing her back was actually part of testimony? 

Does this mean Nardole eventually dies as well on the colony ship?

Everyone dies eventually. Bear in mind that was the Nardole of the year five billion.

And with Nardole being a cyborg really, he wouldn't age that much, if at all.

Oh I see. Guess that makes the 12th doctor the only one to have all of his companions deaths either die on screen or receive confirmed fates.