Using JR pass to travel to Tokyo

Hi, We are planning (4 adults) to travel to Tokyo from Osaka on 27th Dec this year , Plan is to take earliest train from Osaka and get back to Osaka later that day if possible otherwise next day morning travel to Hiroshima from Tokyo. My ques and concern is that if we buy JRPASS, what are the chances of getting seats to travel on Shinkansen on that day (Hikari or Kodama, not too many trains in the morn) as the earliest we can reserve seats is 25th Dec (when we land in Osaka). Otherwise, is it a better option to buy tickets for Nozomi ? As we are planning to travel to Kyoto and Hiroshima also, would have preferred JRPASS. Is there any other pass that will cover travelling among Osaka, Kyoto and Hiroshima? Finally, Is it possible to use JRPASS to cover any fare for Nozomi (such as basic fare)? .

Many thanks.