If the pilots, crew and passengers had all passed out from an unexplained result of gradual de-pressurization, and ground personnel were somehow aware of this scenario, would there have been anything that could of been done to save the plane and people before the fuel ran out?
No. If the crew and passengers have all passed out due to lack of oxygen and the plane has not been commanded for a descent, they will never wake up. This has happened before, e.g. the Payne Stewart fatal accident. In cruise flight an autopilot is only commanded to perform lateral navigation and absent instructions from the crew to initiate a descent it will fly to its last fix and then maintain its current heading, all the while maintaining altitude. When fuel is exhausted it will maintain altitude at the expense of airspeed until the stick shaker activates, disconnecting the autopilot. Absent the pilots input, the stick pusher should next activate to forcefully prevent a stall. You'll probably get some oscillations in pitch but ultimately a steep descent that even if the (literally) brain dead crew could somehow wake would not give them time to act.
While ground crews can get data from the airplane and can attempt to contact the crew, they cannot remotely control the airplane so the best they could do, if aware, is watch. They could perhaps get a fighter scrambles to intercept and visually inspect the cockpit, but that still would not allow them to alter the outcome (just verify the scenario).
Note that this scenario is much more likely to happen in an extreme depressurization event in which the crew is unable to don O2 masks in time. A slow gradual depressurization will give plenty of warning as the cabin begins to climb and automated sensors start complaining. In the EMB-145 you'll start getting aural warnings when the cabin exceeds 10,000 ft pressure altitude and somewhere shortly above that you'll get master caution alarms and automatic pax 02 deployment. These altitudes should give the crew ample warning before becoming hypoxic (you can reach these altitudes for limited time with no supplemental O2 in a Cessna 172 for example). Even at cruise, it only takes a few minutes to get the plane back down to 10,000 ft and as such a slow gradual depressurization should not pose a threat unless multiple warning systems are malfunctioning.