Flooding is a distinct possibility with a power outage or the return pump failing. The only solution I can think of is to keep the water level in the sump low enough to accommodate all the water flowing in from the main tank when none of the water is being returned to the main tank. Might not be possible with such a small sump tank, i.e., you may need a bigger sump tank to do that. To avoid floods, an overhead sump might work - pump the water to the sump...during a power outage or pump failure, no water leaves the main tank, no floods.
Actually, I always leave enough room in my sump tank for all the water that COULD drain from the main tank when no water is being returned to the main tank. Easy Peasy that way.