Sure.
1. iMovie autosaves all the time, not sure how often, but I think it's everytime that you change something. The only way to experiment is to duplicate the project, and try out stuff on the duplicate.
2. If you want to move a project to another computer, you need to take all the appropriate Events for that project as well. Probably the easiest way to allow for that is to import all your movies to an external drive.