replace 10 tbsp of butter with duck fat in spinach casserole?

That seems like an incredible amount of butter or any fat for a casserole. I'd be leery especially wrt the duck fat bc it can easily overpower any other flavors you have in the dish.

Not sure what your question is. Yes, you can sub duck fat for the butter, but you will have a VERY different flavor. Your dish willt aste very "ducky." I imagine it will be delicious, though!

I would do it! 10 TBSP to 10 TBSP!