There are times where the intent is to rapidly shed energy and systems/procedures designed for that (spoilers, speed breaks, slipping the aircraft). I suspect though that implementing any kind of efficient regenerative capability during these times will be something that is developed later/as an afterthought initially. That being said dismissing the idea out of hand is probably wrong. If this system is used during takeoff and climb and would be needed for landing/go around then I imagine they would want minimum energy levels in the batteries so there is probably going to be some system to tap the generator to give them an ability to have minimum energy for landing. Why tap the generator if you can scavenge along the way or just build in robbing power from the prop as the connection to get that power. This system probably naturally beefs up the generator substantially on these aircraft to give them essentially this exact capability. Hmmm.. as I type, the more I think it may happen very quickly honestly. But I'm not an aerospace engineer so this is all a guess!

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.