I don't really understand your post -- what's the linkage between being 10K short of Platinum and not being able to upgrade the full itinerary during the summer? Are you saying that, because buying J won't get you to Plat, you would prefer to just buy Y and upgrade?

Another option is a mileage run. I am seeing NCE-MCT round-trip in biz for EUR 1,972 on 10/18-10/21. Seems like a great fare. This would get you what you need for Plat after your 21K trip and be a worthy vacation (although, living near Nice, you don't need to fly to go on vacation ).