Best lenses for Japan/South Korea

If it is rare you used the 10-20mm then skip that.

Yes, unless you purposely seek out wildlife ...

You seems in doubt with 18-135 and you say you don't take much of buildings.  Since you enjoy candid maybe just take the 70-200 if it is ok to you.  Then you can just take the 18-135 or get a Tamron 17-50 as that allow more bokeh and isolation and low light as you walk around etc.  If that medium zoom doesn't work out, leave it in the hotel.  You live and learn and a better photographer for next time ....

I think that is better than going into primes if you are unsure.  It also limits lens swapping.  A 35mm DX is good walk about candid lens approx to the eye but then if you want a little bit different you cannot just zoom.  And if you did infact like the wider lens with the skyscrapers ...