proposed offline mode
There have been times that I have been without Internet, but would have used duolingo if I had the chance. The most ironic way I lost my streak was on a trip to Germany, where I was using german much more than just on duolingo. and for part of the trip I was without Internet acess.
I would recommend an option to download lessons for use when offline. And when your are able to connect to the Internet again, it could then update your streak.
I got this idea because one of the tools I use in addition to duolingo, is Memrise, and that allows you to download lessons for offline use.
The iOS app downloads the next few units, though exactly how it makes those decisions I'm not sure. I just know I've finished a row and been able to move on to the next. You can't "practice weak skills," test out, or regild any skills that aren't adjacent to your current unit.
As for the streak, when you reconnect it should update. I was too paranoid about my streak to see what would happen if I reconnected after midnight, but maybe someone else knows? I also don't know what happens if you use the app offline for more than a day.
No, you still have to connect before the end of the day's streak cycle for those points to register and continue your streak into the next day. Don't wait until the last minute either. Some phones have glitched and Duolingo doesn't register that you've completed lessons in offline mode. You'll want time to log points in online mode if that happens.
I don't know. And, I don't know anything about computer programming. I'm not sure if it would make a significant, positive change in the number of people who continue their studies with Duolingo for them to invest the funds into writing the programming for it. So, your guess is as good as mine as to how likely that is to happen.