Advice needed on optimizing and reducing the complexity of a large FCPX project

The functionality to collapse the multicam project is not available in FCP as it was in the legacy software. I encourage you to make a feature request for this. It is a very useful and important function for multicam work.

You can get back to the original clip but I haven't found a quick way.

Once you match frame the multicam shot to its multicam clip in the browser by pressing shift+F, note the timecode, then double-click the multicam clip to open it in the angle editor. Press control+p and enter the timecode you noted. That will place the playhead at the correct point in time. Then select the angle that contains the shot you're trying to match frame to and move the skimmer so it lines up with the playhead. Press shift+F to match frame back to the original clip.