Unless your flight is significantly delayed, you'll easily catch that train. (I assume you're looking at the departure time from T2?).

All seating is assigned. Make sure you get into the right car. The train will split into two parts at Tokyo station. Only one half of the train goes to Shinagawa.

If your flight is greatly delayed and you don't make it onto that train, you might consider forgoing the JR pass and buying Nozomi tickets. it depends on your travel plans. Cost of a return trip to Osaka is not that much greater than the cost of a JR pass. (And you can get your exchange voucher refunded, minus an administration fee on your return to your country).

There are no sleeper services from Tokyo to Osaka that I know of.