I'm thinking the "downloading forever" problem may be related to condensation inside the camera electronics.
It seems to happen most often after a filter change (but not always).
It seems you can recover (without power cycling the camera) by turning the TEC off for about 30 sec., then on again.
So the good news is I can recover, without power cycling the camera / disconnecting from SGP.
Bad news is I don't know how to automate this as a recovery step using SGP. Is that possible (turn off TEC, wait 30 sec., turn on TEC, restart sequence when temp reaches set point)?