I'm in the nuisance wildlife and pest control industry. The problem wasn't in the ingredient. As you have found out, it is still used in professional products. It's in the shape.
Mice are haorders. They would find those pellets and hide them all over. Corners of closets, shoes, pillow cases... all the places you don't really want mice poison. A result of this is that whole container of Decon would only kill a couple mice. It was a lethal dose for many, many more. Imagine the issues if deposited in a crib. Kids eat everything.
The solution was to change the shape. If it's secured such as on a post, the rodent can only nibble on it. It only takes the lethal amount. Therefore a 1 oz block can kill up to 25 mice. Much less rodenticide out there, many more rodents killed.
If you can seal the building, mice won't get in. It's very easy to trap them out once you have stopped new ones from coming in.
How can Bell Laboratories sell it if d-con can't? I did go out and buy some traps at tractor supply i will give them a try.