LDL stands for "Low-Density Lipoprotein," but those words don't really mean anything to people besides doctors. All you need to know is that LDL Cholesterol is BAD CHOLESTEROL. It's HDL Cholesteral...
Could be either, depending on the context. Formally known, would imply a formal Title, such as The Duke of Edingborough. Formerly known as, implies a previous name or title, such as, formerly known...
I assume you mean HCl6. There is no such thing as L at this time. There are countless atomic combinations that are not known. Why would you think that this one might exist?