Your setup and temps/humidity sounds great, but without seeing it in person it is hard to tell if it was your husbandry. (BTW: your humidity can be above 50-60, don't ever take out the water bowl. just drill a few holes in the lid.) It definitely sounds like a RI that took his life. It sucks, but this stuff happens. Don't get down on yourself, from what i read above you did what you could and are informed. As for that vet...find a new one. Just my guess: Mid March to NY the package went through some cold weather on it's way to you which started the RI. You probably missed some earlier signs such as bubbles before the mucus. The RI kept developing. That combined with the poor advice from your vet eventually led to his death. Just my two cent guess though. Don't give up!! Contact LLL and see if they will replace. If not go with reputable breeder and look for a ball in the 300+ gram range that is more established.