Glad to know you persisted and continued playing!
As for your questions:
Correct. To get this "secret" ending you need to get all the endings. The route map should clue you where the change will take place (but we guess you already figured that out).
As for what this event entails...well... you'll get to see a funny and quirky exchange between Isaac and Elijah, and then a promise of the brighter future - or just a sweet lie.