Don't worry about it! The Group Q&A is very relaxed. It's you and a few other applicants talking to 1 or 2 grad students for each lab accepting students. The individual interviews you also shouldn't worry about, just 15min 1-on-1 talks with each professor taking students. Just try and prepare a lot of questions beforehand and you'll be fine!
I'm not really sure because School Psych is under the School of Education. I also have no clue if there will be additional interview days or if they're still contacting people
Berkeley sent out interview invites the last week of December and their interview day is scheduled for next week. However, I have no clue if there will be additional interviews afterwards or if there's a waitlist of sorts