Rosh has a long O so it sounds like "roach". All the "A"s in Hashanah are short - so if you say "ah ha" - that's the right vowel sound. So: Roash Hah-shah-nah is a clue to pronunciation.
They're both what, rather than who. Yom Kippur is the Day of Atonement, when observant Jews fast and believe that, if they've made sincere efforts to put right any wrongs committed over the previous...