[MMM]

It is Kodama, not Kodomo. The price difference between the three lines is pretty minor. Most people take the Kodama or Hikari if they want to get off at a more minor station. The shinkansen runs several times an hour, and at this time of year I can't imagine the trains being sold out. Most people buy their tickets with cash at the ticket counter, and if one train is full (a rarity) the next train leaves 10 minutes later. There is no chance of you getting stuck in Kyoto for more and an hour, so you have nothing to worry about.