Yes, I think you are correct. I know that the options in preferences has changed with iterations of iMovie 10 but don't recall which version added the render files deletion choice. It is possible to open the library bundle and manually delete the render files but it is risky as you can corrupt the library if you are not careful. I wouldn't recommend it if you are not comfortable with doing it. If you do want to try it post back and I'll give you the instructions. Deleting render files does not affect the projects you have in the library. Given your versions of iMovie placing your library on an external drive is probably your best choice. As Rich pointed out you open the library with the version of iMovie that created it or a later version not an earlier one.