What does "assist" mean? Alibaba dual-use consumer goods, US-style weapons transfer, or PVA 2.0?
What does "serious trouble" mean? Feb 24th border restored, NATO entrance in war, or Moscow under threat?
It should work both ways, if Russia requests China to allow Russian bombers to attack US military bases in Asia-Pacific using Chinese ports and Chinese airbases, China would have to allow that. Why would China be dragged into a war this way? China would automatically be considered an enemy combatant by US... unless China was already at-war with US, why would peace-time China allow that?
Russia would be opening itself up to US/Japanese retaliation against Vladivostok in that case, what does Russia gain from this situation by allowing PLA to bomb Japan using it's territory (unless it was already at war with US/Japan to begin with)?