lilmod ulelamaid – I’m sorry, but I am not as familiar with the Halacha Berurah than I am with Yalkut Yosef. Unfortunately, I am not well enough versed in either of them to tell you the differences. Best to consult with a knowledgeable Sefardi. As far as whether you can buy them individually, the answer is yes you can buy individual volumes. (I’m not sure about Yabi Omer though).
As far as your question about Ashkenazim learning Yabi Omer, Yabi Omer is a sefer to study. It is not meant as a quick guide book in halacha. Rav Ovadia’s Bikeus was vast and he brings down just about every halachic source out there (including all the Ashkenazi poskim). He shows you everybody’s approach to a specific halacha, where the sources are that they are drawing from and who he leans towards in psak. It is an excellent starting point for anyone building a chabura or wanting to have a full grasp on any given topic. There just about no topic I have not found him discussing. It is the sefer that really helped me see the greatness of a man that others foolishly looked towards with political eyes. He was certainly one of the greatest Talmid Chochom of our Dor.