Regarding the Inyan of throwing away shells (in the gemara, daf 143a and your your Answers to Review Question # 2)(d), I saw that the Shulchan Aruch paskins (Aruch Chaim ?180:4) that it's permitted to throw them out by hand, but one should not step on them or throw them in a place where people will step on them because (chas v'Shalom) it brings poverty to a house.
I was struck by the Shulchan Aruch adding such an eitza to the halacha, but I was not able to identify the source for the Shulchan Aruch to give us such an eitza. Can you help??
The Halachah which you saw in Orech Chaim 180:4 is actually talking about only bread crumbs ("Pirurin"). That Siman in Shulchan Aruch is not discussing Hilchos Shabbos at all, but rather comes in the section entitled "Devarim ha'Nohagim ba'Seudah," proper conduct at a meal. Thus it is certainly appropriate for the Shulchan Aruch there to add such a piece of advice, since that whole section, in addition to the Halachos that are relevant to a meal, also discusses the Eitzos and proper conduct at a meal.
In fact, when the Shulchan Aruch writes the exact same Halachah (that it is permitted to throw out crumbs) in Hilchos Shabbos (OC 308:27), he indeed *does not mention* the Eitzah that he mentions in OC 180:4! That is consistent with what you wrote, that it is not normal for the Shulchan Aruch to add such Eitzos. The Mishnah Berurah there (308:112) refers us to the Shulchan Aruch in 180:4 so that we see the Eitzah.
The source for the concept that stepping on bread crumbs brings poverty is in Chulin 105b (a little less than half way down the Amud). The most simple reason given is that since bread is a dietary staple, throwing it in a place where it can be tread upon shows a lack of respect for one's livelihood. This would not apply to shells and peels.