The Russians might intervene not because they are buddies buddies to the Chinese but because they think for whoever is attacking the Chinese, they might be next in the list.
As I said, that would require China being on the receiving end of an invasion. Most of the likely conflicts over the coming decades would be limited operations - such as a naval clash with Japan. The only big thing would be war with Taiwan, which would be mostly limited to the island itself. Regardless of whether or not the US get involved, China would not suffer an invasion. There would be absolutely nothing for Russia to fear, in regards to it being "next on the list". So it would not get directly involved in such conflicts.
China getting a bloody nose isn't good for Russian armed sales, because the implication is verification that Russian arms suck and if the Chinese rearm, they won't do it with Russian stuff anymore. So their global image is bad and the Chinese will buy even less.
Now that's complete nonsense. China has a fair amount of old equipment. Now if the new Russian gear couldn't do the job, that would be a fair statement. But for the Chinese to blame Russia because it's newer purchases didn't make up for the rest would be ludicrous.
Besides its not just arms sales, but the trade of all forms of goods across the border. Russia also sees China an important customer for its oil and mineral resources and China sees Russia as an indepensible source for that.
Erm, there are
other countries in the world that would buy Russian resources, you know. And a loss of trade wouldn't be worth attacking the US/Japan over - that would affect the economy even more.
Finally, having foreign forces stationed in northern China and facing the Russian southern border is something the Russians will not like to see.
Why are you talking about having forces stationed in Northern China? That is so unlikely it doesn't warrant even considering, unless China had gone into meltdown and required UN peacekeepers.
While this scenario is not possible in the short term, in the long term it may be. The countries you may consider benign today may be not be as benign in the future, especially when resources run short. The Russians and the Chinese are thinking in the long term. Right now there is no need to form a power bloc but if power blocs are setup against both, then even unlikely bedfollows can form one out of necessity.
Thing is that if any countries became aggressive over resources, China would be one to consider going that way. The Russians could have to worry about China, given it's the one power on its border that could cause it trouble. So just as you could theorise about China and Russia joining together out of necessity, one could theorise about Russia forming an alliance against China. Of course Russia isn't worried about that at the moment, but neither is it worried about other countries doing the same.
I prefer to think of more likely scenarios, because if you start to hypothesise about what
could happen way off into the future there are so many more variables, as I pointed out in the above paragraph.