Moving Soviet troops into an invasion of Japan

Could Soviet forces have crossed the Sea of Japan to join an invasion?

Yes, if Allied landing craft carried them after ports had been secured and the initial lodgment could be supplied by merchant ships.

The shipping solution depends on the campaign's sequence. British and American forces would first establish a lodgment; once a captured port allowed larger merchant vessels to supply it, landing craft could be released and sent north for Soviet troops. The proposal depends on Allied amphibious lift rather than an independent Soviet cross-sea capability. Captured port capacity is the trigger for reallocating scarce landing craft to the coalition's next movement.