I think the 9:25 flight is the last scheduled flight of the day into NRT T2, so I don't think Immigration and Customs would be too busy at that time of day. But I have no first hand experience arriving late at NRT, but since the last Keisei train is at 22:41, thats over and hour later should be plenty of time (assuming the flight is on time etc).

There are also limousine buses that go to Tokyo Station, the last one leaving at 23:00 at NRT. The last train from Tokyo Station to Shinagawa Station doesn't leave until a little past 1am so you shouldn't have much problems arriving at 9:25 catching any option train or bus.

For the trains, advance tickets really do not matter as both the Keisei Trains (non Skyliner) and the JR Yamanote Line are both non reserved seating regular commuter trains. Getting on board at Narita airport you should have plenty of choice on where to sit.