I didn't apply to Harvard but from inspection, it looks like most people have heard back already.
http://www.physicsgre.com/viewtopic.php?f=3&t=99428
You could also look through the results page on this website (Top tab, search "physics,harvard").
I'll be applying to HepEx as well this upcoming Fall semester, however I'm actually interested in it. I don't have any experience either. HepEx seems to be quite specialized as the toolset you'd be using is usually specific to each experiment. Perhaps learning C++ and ROOT would make your resume stick out more. I don't recommend applying to something you plan on switching out of though.
Although not all Astro programs require the Physics GRE, in your case it's probably a must have to prove you're knowledgable of undergraduate physics. If you score high enough on the exam, admissions cannot really question your knowledge base. So try to apply for these programs with a high PGRE.
Definitely possible. You just need to do a bunch of practice problems! I think the "Official GRE Quantitative Reasoning Practice Questions Second Edition" might be of value since they have full explanations for all 150 practice problems.