The problem with SIN-ICN-NRT is that there is a 4 hour layover in ICN which, to me at least, is too much.

I know the OP said he wanted to depart in the evening, but in order to get a decent seat on the rather long flight, personal recommendation would be actually the SQ or NH morning flight - SQ flight has the Spacebeds, and NH regional seat will also be fine for the daytime flight.