wondering if I could use orange flower hater instead of the rose water and, if so would I use the same amt. Also would a different nut be better?

If I recall, orange flower water is very floral and doesn't much taste like oranges. If you want to try it, yes!-- I would simply swap the ingredient and use the same amount. When in doubt, I always err on using less than more since floral flavors can quickly overpower.

My guess is orange flower water would work well as orange and chocolate pair pretty well. Taste your orange flower water and use your best judgment how much to add. I would probably start with 1 tsp. the first time around since orange isn't supposed to be the dominant flavor in a Mississippi mud cake anyhow.

As for the nuts, they're totally optional. After all, they are just sprinkled on top and traditional Mississippi mud cake recipes don't require it. You could keep the pistachios or switch to a more American nut like pecans, walnuts, or almonds.

Basically, it's your call how to add these optional ingredients based on your palate and of those who will be joining you at your dinner table.

Best of luck.