If I understand the top of Mo'ed Kattan 23a correctly, it says that one should not say aggadah in a mourner's home. (The phrasing immediately after says that they did this after Rav Chananiah ben Gamliel's death. However, I understand that this was an exception and not another opinion that everyone may do so.)
I have seen in almost every moruner's home during shiva that the rav or someone else recites a mishnah or some other halacha between mincha & Ma'aric in teh mourner's home. If it is, in fact, halacha that one may not recite Aggadah (I assume Divrei Torah is the same idea) in a mourner's home, how are they allowed to do this?