It should be, but it’s important to remember that there are still going to be bugs found and changes made that could be disruptive. That could mean that some of your lesson progress is lost. You will not lose certificates as a result of any changes. Even if the certificate were to stop appearing on your profile due to a code change, you will still be able to resubmit the same projects. The projects are the only thing that is required for certificates.
Totally your choice. Are you comfortable with the possibility that you might run into some errors or bugs as a beta tester? How far have you made it in the stable curriculum? Feel free to try a couple of the exercises in the beta course and see if you like it better. In either case there is no need to repeat anything you’ve already learned. If you finish the Responsive Web Design section, don’t repeat it. Move on.
Mostly in the teaching format. Some content will be more up to date. Lessons for some specific topics have been added, moved, or changed in response to the feedback we’ve seen over the years. Take a look and see what you think.
The whole curriculum is being worked on as part of freeCodeCamp v7. This is a long and slow process. Once the team thinks that another section of the curriculum is ready for public testing, you’ll see an announcement. It’s safe to guess that the new RWD material will have been moved to the main curriculum by then.