How to edit .imovielibrary extension as project?
Found the answer. You cannot open an .imovielibrary and open it as project from the first interface. It will default to opening the library of finished movie clips and allow you to drag into the timeline (which won't include any of your transitions, etc available for edit).

Instead, you need to click the Return to Projects button on the top left (which it will then ask you to save your current project. Save it then delete it). 



You will see your project file there. It won't default (at least it didn't for me) to opening it as a project, instead it will default it opening it as a playable movie. You need to click the ellipsis button under the project thumbnail and explicitly tell it to open as project: