There would never be a reason to use windmilling props in flight for electrical generation. Even when descending, airliners like the Dash 8 need at least some forward thrust in order to maintain control and stay on the glide slope. They only use reverse thrust for a few seconds during the landing roll.

I've seen references to this capability in reporting elsewhere. It's less useful in an aircraft than a car because you can gradually descend, reducing power proportionally as you're preparing to land. But I'm guessing this is part of where the claimed 30% improvement in efficiency comes from.

After they land, they usually sit around for a while. Not really super important to charge the battery on decent I wouldn't think. Would be interesting to get rid of the battery entirely (or reduce the size) and have a beefed up magsafe plug that was attached during lift off, and came unplugged once it was at cruising altitude.