Ultimate responsibility for ensuring all statutory matters are dealt with and the wishes of the council are carried out rests with the council so I agree that the evaluation should be carried out before any agreement to hire (or fire).
I also agree that proper contingency planning dictates that provision be made for periods without clerk cover, for example through accident, illness, etc. Whilst most parishes can probably manage for a short period of time, the complexities of the job are such that for many parishes a locum may be required and these come at a cost. You may be able to purchase some insurance cover for absences caused by accident or illness for example so its worth discussing this with your insurers too.