The process of appointment of replacement officers, or indeed any committee member depends entirely on your constitution. If you don't have a constitution, then you need one, and fast.
Any normal constitution should make it clear that you can appoint replacement members as and when needed on "holding" basis. The purpose is to ensure the committee can function, which it cannot either if it is not quorate or if key officers are missing.
If the constitution says an EGM is needed then it is needed. However if the constitution says that then, imo, it seems over the top bureaucracy for an allotment society, and I would personally be interested in rewriting the constitution to remove that need.
When I became chair, which was mid-year, I was appointed as interim chair. The protections that were in place were:
- the remaining committee members appointed me
- subject to ratification at the AGM
- all plotholders were notified at the time of my appointment
- any plotholder thus had the opportunity to complain and perhaps even request an EGM.
They didn't complain, and at the AGM I was properly voted in. That, to my mind seems a sensible & pragmatic process and one that should happen for any officer.