My understanding is that you could selectively attack just Italian forces if Germany hadn’t declared war. That strangeness has been eliminated in the new BM version which prevents Germany from marching into former Russian territories without DoW.
Nevertheless I am having a related problem in a League match where Russia is attacking an original German territory with both German and Italian forces. Italy is currently at peace so TripleA assumes that they are not aiding their German friends. It has huge impact on the game so this is a critical rule argument.
For naval forces you can indeed selectively attack a country A which your at war with and leave country B out of it.
For land this is not the case and you cannot attack said country.
Russia can declare war if it is attacked by Any Axis forces in europe ( italy or germany ) or at the start of round 4. So russia can just declare war on germany in this situation.
And yes this allows germany to get their NO for 1 turn longer then normal which isnt a real big deal.