Yep great idea
I can fly BA ABZ-LHR-HND
then JAL HND-HKD (OW

)
Then onto an Aeroflot to UUS
I think that is definitely the best option and 95% of the journey will be OW (and no risk of being stuck in Siberia)
EDIT: There are no flights to Sakhalin from HKD but there are from Sapparo Chitose, so will JAL from Haneda to Sapparo Chitose, then SAT Airlines to Sakhalin