... how did our beloved cousin return?
None of those other Supergirls ever happened. The new Kara is unrelated to the protoplasm and is Kal-El's cousin. She came to earth a few months back in the Superman/Batman title, then spun-off into her own. I believe that her origin and early adventures have been collected in at least one
graphic novel. The link to this item also shows a lot of customer reviews containing plot summaries.