These are all round-trip fares. Now that I look at delta.com again I see where I could have searched for Y class - rookie mistake! Y is $5017 for the same itinerary though. Big jump.
At least one of my main concerns here is about timing - should I call and try to upgrade now, or wait until I'm sure I will be flying. $150 (per person?) seems like a reasonable price for that option.
How does one check for upgrade availability (the seat map does seem mostly empty) - just call Delta?
I was able to pick my preferred Comfort+ seat so if I am stuck in economy it's not the end of the world. Just a very strong preference for business.