A power not at war cannot move land or air units into neutral territories. This question relates to when a state of war exists between some powers but not others.
First - If China is at war with both G & J, but R only with G, can R forces enter China?
Russian forces can enter China but Russia needs to declare war on Japan before.
See for Russia:
Political Situation: The Soviet Union begins the game at war with no one. The Soviet Union may not declare war on any
European Axis power before turn 4 unless first declared war upon by a European Axis power or London is captured by an
Axis power. It may declare war on Japan at the beginning of the Combat Move phase of any of its turns.
Due to its separate treaties with Germany and Japan, the Soviet Union is in a unique position in its relationship with the Axis
powers. As a result, if the Soviet Union is at war with Axis powers on only one map, it is still under the restrictions of being
a neutral power (see ‘Powers Not at War with One Another,’ page 15) on the other map. In other words, a state of war with
Japan lifts those restrictions from the Soviet Union on the Pacific map only, and a state of war with Germany and/or Italy lifts
those restrictions on the Europe map only.
and for Japan:
Political Situation: At the beginning of the game, Japan is at war only with China. Japan considers movement of units into
China by any other Allied power as an act of war against it. …
So in your example Russia being at war with Germany only leaves Russia being neutral on the Pacific map. That would prevent Russian troops from entering China. If Russia declares war on Japan before, Russia will be no longer neutral so Russian troops may enter China, then.
Second - If the answer to that is yes, we have a possibility of J attacking a Chinese territory containing R units. In which case are those R units ignored, as with a sea battle?
As a state of war has to be established between Russia and Japan, Russian units defend against Japan then.