The three remaining councillors (excluding chair) should try to act. At least two of them can requisition the chair to hold a meeting, and if the chair does not act within 7 days, the councillors can call a meeting. The councillors can determine the agenda for this meeting.
To some extent, the chair is still in control, since the meeting will not be quorate unless he attends along with the other three councillors. So the meeting needs to be arranged to suit all councillors and pressure (personal? publicity? press?) exerted to persuade the chair to attend.
It should then be possible to pass resolutions to ensure the bank mandate requires two signatures, to set a programme of regular meetings, to appoint an acting clerk (a volunteer councillor can do the bare minimum to ensure meetings go ahead and make essential decisions), to instigate co-option of additional members, to decide anything else considered important.
Also, bear in mind that the next meeting of the council ought to be its annual meeting, which has to take place during May. The chair ceases to hold office at that meeting and a new chair has to be elected. If this is not done, the council is no longer properly constituted.
The advice to involve the external auditor is good.
If the above fails, then there is no alternative but to press the principal authority to appoint additional councillors as quickly as possible.