The chair (or any other Cllr) nominated by resolution can act as the RFO if so approved by the council
The RFO can make payments approved by delegation or resolution with any falling outside of pre established delegation requiring the approval of the council as would be the case for a permanent incumbent of the RFO role.
It would be for the council to decide if engagement of a locus clerk / RFO was necessary dependant upon the circumstances but it is not a simple default to locum clerk if there is a suitable and willing candidate from within the council or even the wider community.
Whilst it might be appropriate to engage a locum clerk, it is not mandatory