]After all these years I thought it prudent to check my understanding of why Lydia's elopement is so dreadful. I have always assumed it is because she lived with Wickham before being married. However the wealth of info and analysis on these pages suggests I might well have missed something?

It is dreadful that they lived together before marriage. Even now unmarried poeple living together as if they were married is considered not right by some people if not by all.
Also, Lydia running away with Wickham involves not only themselves but their whole family, it will be talked about by all who knew them.