Is it possible to set up Garmin 1030 so that the arrival time or the average speed is right?

Depending on the site you are using to create the route, there should be an option to set an average pace. If you set the average pace to give you the elapsed time you need then the Edge will report time to go  and time ahead/behind. I would recommend trying some shorter test routes before the day so that you get a feel for how this works and what data fields you want to see. E.g Estimated time to destination, estimated time of arrival.

I have tried creating routes with different web sites including, just recently, Garmin Connect. Despite setting a pace and then seeing the expected time and average speed in the stats for the course on my Garmin Edge 1030 before choosing to navigate by it, I get a "time to destination" when riding the course that is completely at odds with the pace I've set. Depending on the course, it varies between a time that would correspond 14 to 15 mph despite setting a pace for the route at 10mph or 17mph. I'm not yet sure whether the virtual partner rides at the correct rate but the estimated time to destination is definitely not being calculated correctly and is not even at a consistent incorrect pace, so I'm at a complete loss to understand it.

I've done a further check and found that routes generated with Garmin Connect, but without specifying a pace, default to 25kph when the summary of the course is checked on the device. However, time to destination at the very beginning of the course is about 4 minutes longer that you'd expect and I get exactly the same time when using the same course set at a pace of 10mph. My conclusions are that 1) Time to destination completely ignores the pace set on the file and tries to use the default of 25kph and 2) The calculation is still flawed because it seems to add a few extra minutes to the time you'd expect. What a mess!

I have an IQ App that will calculate that based on your average speed.  It  is called Distance Plus.  You can use it as a 1/2 width field or full width field.  Look for it by R Bryant.