Under Tokyo station as a place to spend your one night in Tokyo??

Absolutely not!!!

As you're clueless, and this is even a consideration, may I respectfully suggest that you start off your evening at Andy's (be quick though, it does close at midnight, however there will be people moving on from there)
http://www.andysfish.com/Shin-Hinomoto
Yurakucho Station is a short walk from Tokyo Station, you can't get lost as you just need to follow the train tracks.
Go there, talk to the owner, talk to other customers, take a notebook and get recommendations. If that doesn't work out then don't worry, those same rail tracks are on arches and there is a never ending number of bars, izakaya and restaurants underneath them.

You can still return to Rokurinsha for breakfast (as mentioned, it does seem to be "a thing").

If you're still stuck as to what to do, there are a few suggestions here:
http://www3.nhk.or.jp/nhkworld/en/vo.../20151209.html