I think you should book for 2 pax. Otherwise the hotel has very good reason to charge you for the second reason. If you book for 2 pax, the hotel can still charge you for the second person because "second person charges are not covered by the guarantee" outside Japan, but I doubt that many hotels will do that.
HTB.
Thanks for the reply. After reviewing the T&Cs again yes it is stated it won't cover extra person charge.
"The Guarantee does not include extra fees such as extra person charges, except in Japan and on the ANA website where the room price does include extra person fees. "
Ok I'll book it for 2 pax then. Fingers crossed they don't invalidate my claim for some stupid reason.
Edit: Booked 2 pax and worked. Hopefully no extra charge on the day itself!