Ashkenazim as Rama and Sephardim as Shulchan

peace,
 
A. What is the source of the validity that Ashkenazim should rule as the Rama (or as is customary among Ashkenazim, etc.) and Sephardim as the Shul? Or do you think that this has no validity? What is the point of the dispute? I emphasize that I am asking about halachic rulings and not about customs.
[I will also emphasize that the question is regarding a situation where a person does not have a solid and definitive opinion regarding a particular halakha, in which case I assume you would say that he should rule according to his opinion, even against the Rema/Shula].
on. Now I will also ask about customs. On the face of it, the moment we left the lands of exile and came to the land, the local customs of our grandfathers and grandmothers were nullified, and one must rule according to the rest of the rules of halakhah. Or are the customs of our ancestors also valid?
Also, is there a connection between these concepts of local custom and ancestral custom to the first question?
 
Thank you very much.