I made orange-flavored SMBC, and it's delicious, but I'm concerned that the butter is overwhelming the orange flavor a little. I don't want to add more extract, because I'm afraid that it will get to be too harsh and nasty. So I had the thought to ad some little pieces of fruit when I torte and fill. Should I use fresh pieces, or candied orange slices? Fresh pieces sound wonderful, but I'm afraid they might be too tart to go with the icing. Any suggestions?

You will get more flavor from orange zest. I would take some of the icing and use a micro planer to get very fine zest. Blend the zest with the smaller amount of icing to be sure the zest mixes well.

Ditto orange zest, and perhaps a little of the juice.

If you want to be really decadent, add some Gran Marnier - that's usually what I use for my orange cake. I also add a touch of cinnamon, and will frequently pair the orange cake with dark chocolate ganache frosting.