You can’t do that.
Because for all land battles, you must be at war with all nations you are attacking, whether they control the territory, or units. It is only at SEA that you can attack units of powers with which you are at war, and not others.
I would quote you rulebook but don’t have time. Family waiting for movie night.
Oh. Just saw your second question is a different one.
Germans can move into Italian territory that was originally Russian without declaring war because it’s not Russian territory anymore. It’s Italian.
Note that a German sub in 125 nullifies the NO even if Russia is not at war with Germany at the time, but is at war with Italy only.