I use Chicago and I don’t think leaving it plugged in ruins the battery, at least not for me. I was flying in Chicago for 5 hours the other day and I don’t notice a difference. And your iPad won’t overheat as long as it’s inside. I’ve never even had my iPad overheat, even outside :)
You shouldn’t leave your charger in for long times after your phone or tablet has reached 100%. Just keep that in mind. It burns out your battery a ton. I did that all the time with my iPhone 6, and now it’s battery life lasts shorter than when I first got it.
Good question. I’m not sure how I’m supposed to do a 7 hour flight on global. My plan was to cruise, go to sleep, wake up and land. But that requires my phone to be plugged in so it doesn’t die during the flight. But once again, your battery would get killed unless you set up several alarms for you to wake up to unplug and plug back in your phone.
Also, as long as you are using an apple charger, it keeps your battery healthy by discharging and charging again and other complicated things. Apple designed this specifically for this reason of leaving it plugged in. So just make sure you’re using the charger that came with your iPad and you’ll be fine.