Hi, Steve,
As you have observed, the only way to reattach the audio so that it is again embedded in the video clip is to share the project out to file. You might not need to share out the whole project. Possibly you can copy to a newly created project only that portion of the video and audio pertaining to the detachment, and then share out the new project. Then import the shared out merged clip into your master project.
However, why not leave the audio detached? It will couple with the video above it so it shouldn't get out of sync. Besides, I think you are going to end up with a final shared out project of 178GB in any case. That would need to be verified, which you could easily do. iMovie is exporting at less compression than what you initially imported, which accounts for the larger file size. My hypothesis is that once the footage compression has been reduced you will not be adding any more file overall file size to the final product by importing it back in. As I say, you could make a test movie to find out.
-- Rich