有限公司話事人？

The shareholders are the owners of the company. However, the daily operation of the company is vetted or controlled by the board of directors which is formed by the resolution in the annual general meeting of the company. The directors could then get the powers to run the company therefrom.
I think the issue you mentioned as the signing arrangement i.e. A + another person ( Z ) is the cheque or banking arrangement and not the ordinary daily operation arrangement. The board of directors could resolved that A is the chairman of the Board or the Managing Director. And, please note that the office of a director could usually be appointed or vacated by a majority of the Board of Directors or the Shareholders by simple majority. So A, technically speaking, is not 'safe' in absolute term.