Operations (zone) should be monitoring all the differing departments that are required to make a plane depart. Of course they are not at the gate. So the communication should flow from ops to the gate, where the gate agent would use his/her microphone. On board, the flight attendants don't have a direct line to ops, so info would flow from ops, to the gate agent, to either the passengers directly or thru the flight attendants.

Basically, at the gate, information flows thru the gate agent to the customers. Sadly, it is often from ops to the computers to your mobile devices, to you, to the gate agent, as often the CS agent isn't notified of things directly.

Somewhere along the line, crew scheduling should also be notifying ops of issues in their departments, but they are often not operationally the fastest and are often unaware of last minute quirks until notified by others of a problem.

On a mainline plane, if the pilots are reasonably close, boarding is not impacted (assuming the power is on in the plane, which can be turned on by either pilots or mechanics.) It is the flight attendants that are required to board. On express, I've never boarded any plane without at least 1 of the 2 pilots. On non-originitating express flights, crews are usually together all day, on mainline flights, crews often come from all over the place (at hubs anyway.)