You can try out for mock trial and, if selected, would be able to do a journal as well. Law Review is very tough to get, so you'll have to work your ass off and get lucky. Hopefully you do. As for the Health Law Cert, you won't have to do anything in regards to that during 1L. That certificate carries its own requirements, i.e. special classes, seminars, writing reqs, and you will satisfy those during 2L and 3L. If you'd like to do a semester in DC, you won't be taking core Health Law classes (anyone can correct me if i'm wrong). Instead, you'll be 'working' or 'externing' for credit, which can be beneficial. If you can squeeze in all the HLC req's in 3 semesters, and you probably can, then you can do both.
I don't know about participating in journals/mock and doing D.C. so I can't help you there. But there is no problem with doing mock/journal/both and the health law cert.