Updating this post with some relevant news and observations about my Mavic Pro Platinum and DJI Goggles purchase, since my original post a month ago:
Costco price protection: As mentioned above, Costco customer service desk credited me $100 during DJI's limited time June promotion, two weeks after my original purchase. (That promotion ended June 17th, 2018, but keep an eye out for it to return around July 4th US holiday, perhaps.)
Costco return policy: After DJI online support helped me diagnose a faulty remote controller causing close range aircraft disconnections, I was able to return the entire MPP/Goggles bundle back to my Costco for exchange under their 90-day satisfaction guarantee. New gear working flawlessly so far. Extremely thankful I didn't need to go through the DJI warranty return/repair process to resolve.
Noise: As hoped, the lower noise MPP has me much less self-conscious about flying in parks and neighborhoods, people who've heard other drones have even commented on how surprisingly quiet it is.
Size/Portability: Easily took my full bundle (MPP, RC, goggles, spare batteries, chargers, etc.) on a long weekend airline trip, in just my carry-on wheelie and backpack. No issues at TSA, they just want the electronics and batteries to go thru x-ray machine separately vs. in suitcase or back pack. Naturally, it's the DJI Goggles that are the bulkiest, but still very manageable in DJI's well designed Sling Bag that comes in the Costco bundle.
State Farm insurance: My entire bundle is protected from damage, loss, and theft by State Farm's $39 annual personal article protection -- $1300 coverage, $100 deductible -- so I'm no longer interested in DJI Care Refresh.
DJI Goggles: Love them, my preferred way to fly, definitely gives a sensation of flight, and the big virtual screen reveals much more detail than a smartphone (esp. in daylight). I prefer gimbal-only head tracking, to keep flight control fully independent from looking around. I enjoy going back to view recorded flights in DJI Goggles, then I notice details -- animals, people, landmarks, etc. -- I didn't catch when I was multitasking for safe piloting.
Having friends try the DJI Goggles with gimbal head tracking while I pilot from DJI Go 4 app continues to be a lot of fun, still a scenario only available on Mavic Pro/Pro Platinum. (That said, someone on this forum reported Mavic Air now supports equivalent FPV head tracking as Mavic Pro/Pro Platinum, contrary to my original post above, but I haven't read that elsewhere yet.)
Fixed Wing Mode: Seems clever, but so far not as enjoyable since its tradeoff is substantially restricted FPV -- even regular pilots can look out the windows without steering the plane!