Unfortunately, Excalibur seems pretty tight-lipped about how much salt is in their products. We’ve had a few discussions around here along those lines. They are good, solid products, but you’ll have to shoot a bit in the dark if you want to use it.
You’ll probably need to do a little trial and error to find the sweetest spot for fat percentage, but I would start with 25% of the weight of the duck breast. Pork butts average around 25%, and that’s what you typically put in boudin.
I’d season both before and after cooking. Seasoning before you simmer or poach everything gets that flavor deep and mellows things a bit, and then you can tweak as needed after it’s all cooked and ground. One thing I think about boudin is that the seasoning is more cooked in and not so edgy as raw. Plus, you’ll also have that seasoned cooking liquid for mixing back into the ground ingredients when it comes to mixing and stuffing (assuming you’re stuffing)