Hello. I am having difficulty tracing the origins of the specific laws and minhagim of shiva/avelus. Most sources I see do not trace each to a particular gemara or the opinions of rishonim. Where could I find specific information about these original sources and the opinions/shitas of rishonim, and which things are binding halachas and which are just minhagim?? Thank you,

Shalom, Thank you for your question. It is great to see your desire to increase your Torah learning – may you be blessed with every success. How are your Hebrew and textual skills? If you have the ability the best source is to go back to the Beit Yosef (Yoreh Deah, from chapter 340 and onwards), and learn the opinions as he records them. Then to see the same laws as ruled in the Shulchan Aruch. If this is hard for you, you could try (also in Hebrew) seeing the Aruch HaShulchan (ibid), who generally records the sources for each law. The classic modern text that records the sources for each law is "Gesher HaHaim" of Rav Tucazinsky zt"l. I see that it is available in English – but I have not seen this edition myself, and I am unsure as to whether the complete text was translated. If you are looking for a work in English, I am afraid that like you the books I've seen do a wonderful job of recording the "bottom line" halacha, but are not aimed at laying out the sources of each law. In general, the laws of mourning are (more than most areas of halacha) full of later day customs that do not have a source in the Talmud or early Rabbis. For example, the custom of covering mirrors in a house of mourning. This practice, which is found in every house of mourning I have ever visited, has no source in the Talmud, or Rishonim. In fact, its first mention in writing appears to be in the Chatam Sofer, in one of his Drashot from 1832! This is not to say that it does not have early roots, nor that it is baseless – it is a universal custom that has good reason. But rather it comes to highlight the fact that you will not find the sources to all our customs and practices in the classic texts (Beit Yosef and Shulchan Aruch) as you would with many other areas of Jewish law. Blessings.