[watchfirst9]

 It seems you do not know there is treaty signed when Russia take Outer Manchuria.. You logic doesn't make sense. Both sign black and white not because they want to, but because they are in the losing side.

If go by your logic, if Taiwan is an island province of Japan like Hokkaido, and Japan won the war like russia, then yes it is apple to apple comparsion. Too bad also when you are on a losing side like Japan, what you win by force get taken away by force.

If US or Russia claim Taiwan to be their at the end of the war, it would be the same like Outer Manchuria now.

They do not want to is true, but they face reality of four fronts,
then Qing govt was at war with UK/Fra and a big rebel by taiping revolution ongoing.
It is more painful to manchu because it is a place where manchu set begin their Qing/hou Jin campaign it is also other nomadic tribes 
ancestry land.

Reclaimed back is difficult unless you want to denounce the agreement which make one looks very bad,
Also Rus is still nuclear super power impossible to defeat,
Furthermore the population is long replaced by non asian faces already.
Although many Chinese Jap Kor tourists there, foreigners no voting rights.

As for tw it is agreed by us ussr cn govt to return to cn.
You can go to Japan govt web site to read how present constitution are formed 
https://ndl.go.jp/constitution/e/etc/c06.html
base on Cairo Declaration.