The Soviet didn’t have capability to invade Japan, they just had claims to Sakhalin Islands and possibly Korea which they lost in the 1905 war. The A-bomb created a huge vacuum for Stalin to mop up demoralized Kwangtung Army units in Manchukuo.
Okinawa could not be bypassed, a further invasion of Southern Japan could not take place without this logistical island. You need a good staging area and Okinawa and Iwo Jima are both necessary to that end.
Mainland China would not be good because Japan still had credible army units within reach, but had zero naval capabilities…so it was a safe choice to use the islands to stage invasion.