Maybe those questions you should answer or address to the board & management. Sounds like the property manager had enough dealings with the board and managing agent. The shareholder that posted about a farewell party didn't want the board or managing agent involved or meddling in the festivities. Why? You need to address the way the board and management is run, because the shareholders didn't want you guys to be making this decision.
It's up to the board to address replacing the property manager, 490 units sounds like you need an on site managing agent. Good luck in finding the right property manager.

As a board member what information are members entitled to such as master shareholders list, parking list, contracts, hiring of new employees, and communication to our membership? For years our cooperative has been ran like a secret society without transparency. We want to change this image to include all board members in decisions being made for the best interest of the corporation. Loyalty was to former employees who joined the board with their own interests at heart who have resigned without explanation after many years of service. We want to restructure as a well oiled machine with transparency and accountability to our shareholders. Who can we turn to with guidance to make this happen as too much is unknown? Our lawyer doesn't offer advice until there is a need.

Sounds like there were other unknown issues involving the managers departure that would come to light at a later date... keeping the ball rolling and the building functioning at peak performance is top priority now anyway and there are alot of variables unknown that make it hard to answer a question as to if you need on or off site management.. the short term gain of selling the unit can cost more in the long term if you remove a needed service.
~AR