Helicopter requires rover to be near by as means of communications as helicopter itself can not directly communicate. Thus, it originally had a limited time as the rover needs to move on to other activities.
Yes, that is the plan now. It was not the original because…we’ve never flown on another planet before. It was successful so they are changing the parameters after learning more. SCIENCE!
About 600m per flight. Actually it's flight time is limited by motor heat buildup, not battery capacity.
If they want to scout each landing site, that requires doing a round-trip one day to get photos and flying out the next day, which limits it to a maximum range of 300m every two days.
The previous rover, Curiosity has driven an average of 8m per day (usually stopping for a few days to to science then driving in bursts of ~30m/day) Ingenuity should be able to keep up.
Whenever the rover moves on, the helicopter can move to a new landing spot within range.