*The stages of MM’s month*
Mimzy has devised a clever way to take your mind off of Shadow, allowing Shadow time to grow in his fur so that when you stop fussing with the bunnies you’ll look up to see a beautiful, fuzzy Shadow once more.
I think you’re on the right track, and I am very interested in the Organic Oxbow. If they’re pellets, I’d say start them all on them. Toss the icky brand you’ve been using. You could email them to complain (I’m sorry, but plastic and staples in the bag?!) Problem is, they’ll probably send you a coupon to order more.
I think the alfalfa pellets may be a problem if they add too much change and confusion to the mix. I’m inclined to say put them in the closet for a few weeks, or use only as little treats as K&K suggests. Part of the problem with the poopy butts is frequent changes to diet. If there’s a calcium issue, maybe you can sell, or donate, the bag to your vet or a shelter.
The Organic Oxbow may be most similar to the Basics T, so if that’s on its way from somewhere else, it won’t be long. And if the Organics stop the sneezes, stick with them. 
Yes, I suggested a litter change as well. Start with small bags, so it doesn’t cost too much.