Figured I'd give an update.. I've been sick & I hurt my back this week so I haven't been able to give Slippers the attention that I wanted to. But as I suspected, all of the feathers that had previously started growing in before the food change & vitamins, fell out. They did not break, but they fell out. Some of them have "stress lines" in them, and looking into that further, I found that stress lines indicate there was something that disrupted the flow of nutrients to the feather follicle as the feather was growing. I noticed the lines earlier on, and I did notice that it looked different. It wasn't until I did some more research that I recognized what they were. But I have also read that if the feather sheath remains on the feather too long it can also cause marks that appear to be stress lines, but are simply caused by the feather sheath being there too long. Feather quality issues is not something that I know a lot about, so this has been a real learning experience. I've always been happy with the feather quality of our chickens, but he is also our first Sultan. Much heavier foot feathers than our brahmas. He's currently in the house in one of our isolation areas. He has a pretty big area and is on a few inches of soft wood shavings.
The good news: He has had a few feathers grow totally in on his feet, with no issues. He has quite a few new feathers growing in, so I'm very interested to see if they will continue to grow in successfully or if they will fall out. If they fall out, then maybe I need to look into other causes other than the nutrition issue.