There are other cases where there were issues with the exit number. For reference, see this thread: Kreisverkehr Ausfahrten
The angle calculation currently creates more issues in my opinion, but I guess a combination of both will work best. We have this on our list and will improve this further.
This is provided by the GPS sensor in your device and calculated by your device, we simply show this information.
Yes, this would be also possible for us, right now the priority for this is not that high IMHO, it is something we can always add in the future 
.