I can only ans the Hokkaido part.. since that's where I went before and will go again
Check accuweather.com for the latest weather update. But for Hokkaido, being in the artic circle, their summer won't be as hot as SG de la, ard 25-26c max during mid day. And trust me, temp will plunge considerably towards late afternoon and its normal to experience a 10-15c difference between mid day and night temp in many parts of Hokkaido.
You still need to pack thick sweaters along even for summer trip. Themal underwear may be a good idea if you want to stay comfortable outdoors for long period of time in the evening. Rem hor, winds is strong in Hokkaido.
But what I've just said above is based on past yr average. Winter was unusually long this yr and until 3 weeks ago when I was there, average daytime temp was ard 14-17c and I got to enjoy a 4c weather at Memembetsu Airport area upon arrival
Night time temp was ard 4-8c back then