Sorry Ian. I did read it but I wasn’t able to respond at the moment and then didn’t get back to it.
So the LayOut file contains the SketchUp file(s) as a reference although in most cases it will be linked to the SketchUp file you started so when you are editing said SketchUp file, you are editing the original. When you save the LO file the SKP reference file gets updated to reflect the changes you made in the original. When you choose to unlink the LO file from the original SKP file, LO looks inward for the SKP file instead of out to the original. The file still exists. It’s just packed away in the LO file.
As I think we’ve already established, with the viewports referencing only the embedded SKP file, anyone who edits the SKP file will be editing the embedded one, not the original. Think of the LO file as a file folder with the content for the project in it. If you take that folder to your desk, you have all of the references for that file in your possession. If you make changes to it, only the files in that folder get changed. For the most part this works great because when you put the folder back in the file cabinet, I can come along and take it to my desk to work on. Obviously the folder can only be on one desk at a time. If two people want to work on it at the same time, they’ll need to get cosey.
Now there’s a little hitch in this process when you do as you are creating multiple LO files that originally referenced the same SKP file. When you embed the SKP file in one LO file, it is no longer the reference for the other LO files. So you could be working on elevations and change them but the person working on the plans section won’t have your changes. The solution could be for you to open the SKP file from the Elevations file and do a Save as… to save a copy onto your computer. Then open the Plans LO file and relink the SKP file it contains to the one that you just saved. This would replace that files old SKP reference with the newer version.
That process would be workable but you’d need to use caution because if both of you are modifying your versions of the SKP file, at the same time, it gets tricky to combine the updates into a single file. It’s possible to do it but you’ll want to get everyone else out of the room so you can concentrate to make sure everything gets updated.
Exactly what you do with this will take some thought. I believe it is manageable, though.
As for why the LO file didn’t prompt for saving, I can’t say. I haven’t seen that before in my work. If I’ve edited the SKP file and thus changed the LO file, I’ve always been prompted to save on closing. I guess, being a belt and braces kind of guy, I would get in the habit of hit Save before closing the file even if you didn’t make any changes to it.