I've carefully addressed this same issue by making sure my return line is near the top of the tank. That way, when the power goes out, as it inevitably will at some point, the tank doesn't drain beyond an amount I can predict. I'll always mark my sump with a max fill line that matches disaster scenario so when everything shuts off and fills up, I can safely predict the sump will fill just below the rim and nothing bad will happen. A large enough sump container solves this. If the container is too small you'll end up refilling your tank every day when the pump starts sucking air because the water level is too low and it's really annoying.2. if pump loses power or it stops functioning a check valve would avoid back-flow in the return line and prevent sump from overflowing and/or drainage of main tank through the return line. In all the videos I've seen nobody has mentioned this so I assume this is not a problem at all.
That being said, I like the idea of a check valve with two caveats: 1) If it doesn't make a solid seal after a year of slime build up, it can still leak past the barrier and past the critical fill line. Also, if the valve requires water pressure to be open a % of pump efficiency is spent on pushing the valve rather than actual water flow. I just thought of a third. Some of the valve mechanics restrict the actual internal flow capacity. I usually use piping bigger than the pump needs in order to maximize flow with the least resistance but that's been because my younger self couldn't afford any more pump than I absolutely needed. The habit stuck.