1) I've used OneSignal to send push notifications when app is in background or swiped away or killed and signalR to receive a notification when app is foreground.
2) I do not receive notifications post 15-20 minutes when app is killed and notifications sent during those period comes altogether when I unlock my phone using power button.
3) I'm not sure if this is right approach to handle this kind of scenarios, Please guide me if I'm wrong.