You damn well better ENSURE the CC are fit to exceed their duty into discretion!! It is Captains discretion - the implication being YOU (the captain) are responsible to ensure that the CC are fit to continue.
I am not saying discussing it with individuals, one by one but, as a group they can be told that duty needs to be exceeded and if they are not fit then they should step forward. If they want to offload themselves then they can do so.
Should anything ever happen while in discretion, if you have NOT ensured they are fit YOU will be held legally liable and quite rightly so!!!!
You want to bandy around "I'm the Captain" then you had better be ready to act like one.