All that role flip-flopping sounds like a recipe for confusion but I suppose you get used to your airline's way of doing it. What does Airbus say?
My airline follows the manufacturer's guidelines (Boeing in my case) i.e. either pilot announces the problem, PF handles, takes radio and initiates QRH actions etc. PM carries out QRH actions/troubleshooting/monitoring etc. This happens regardless of who is PF (Captain or FO) when it goes south. Seems to work fine. Obviously the situation may dictate that there should be (or the captain may prefer) a role reversal at some point.
I think suggesting that an FO wouldn't challenge/warn the captain if he was about to run the incorrect checklist or activate the wrong switch/button is being unfair to FOs (at least in the company and area of the world I work) - in any decent airline with a good training department, cockpit gradient and CRM are pretty optimised I would humbly suggest.
We're always learning though - our last recurrent CRM course was all about effective monitoring as it's an area that, as you suggest, is sometimes identified by audits as having scope for improvement.