Here it is:
UIAlertController okAlertController = UIAlertController.Create("Selected", SegmentedControlOrderStatus.SelectedSegment.ToString(), UIAlertControllerStyle.Alert);
okAlertController.AddAction(UIAlertAction.Create("OK", UIAlertActionStyle.Default, null));
PresentViewController(okAlertController, true, null);
And before you ask, no there's no direct Toast equivalent in iOS. You'll have to roll one yourself or use a library.
Yea, remember that especially when you're doing asynchronous calls inside a Task.Run() and it does not return at all. That means there's an error somewhere during the async call however since the async call is running on a different thread then you can't see the error in the debugger since the task terminates itself.
Go to the ViewController properties, under the Layout section, check Hide Bottom Bar on Push.
However, calling NotifyDataSetChanged() (and other related methods) wouldn't work if called in Activity (Xamarin things.. probably because the reference is lost).
So, you'll need to put the list inside the Adapter instead and call the methods over there instead.