On my 48GP036090311, the main fan (that moves air through the ductwork, not the condensing unit fan) began to run constantly, even with the T-stat disconnected. Don't see any evidence of physical damage in the control panel section. Is this a relay failure, or something in the controls?

It could be the relay, or the red and green wire are touching some were between the Tstat and the A/H unit. If you want to see if the relay it's the problem, disconnect the green wire at the A/H unit, if the blower keeps running, its safe to say that the relay it's the problem

If the system has a flame failure or if a limit is open the blower will run all the time. It should give you a code, if you count the blinks of the light on the control board.

The red LED on the control panel is blinking a 7 code. An internet source indicates a "rollout switch failure". Does this make sense, especially if the T-stat has been disconnected and the AHU still runs?