How is it possible for Maps in iOS 13 to have location permissions set to “Whilst Using” but provide navigation instructions in the background

I think this is how to do it.

First thing to do is enable time zone support within Calendar. Open Settings, select "Mail, Contacts and Calendars". Find "Time Zone Support" under Calendars and turn it on. It should automatically set the timezone to your current location, that's fine.

Then create your event in Calendar. The only important fields for scheduling the event correctly are the time and the timezone. Location for example doesn't affect it. Just enter the correct time (13:00 - 16:00) and set the timezone to a nearby city iOS will recognise like Las Vegas. Once you click Done you should see the event in your calendar with the time automatically converted to the time in your current time zone.

Then once you are in the US you need to remember to manually set the Calendar timezone under "Mail, Contacts and Calendars" to Las Vegas or another city in the Pacific Standard Time timezone.

Then your events should appear with the correct time.

Location Services is under Privacy in Settings. You can scroll through the list to see apps which have used geofences to find the one relating to the icon showing it in use.