how would china defend herself or attempt to assert control over the russian far east which she once possesed before, should war ever break out in the northeast(border region with russia)?. will the navy be of any major significance in that theater? will the type 99 MBT prove its worth against russia?
Since this is a very hypothetical question as most of the border disputes have been settled through deplomacy.
The way I see it is that as shown by the Japanese invasion of Siberia, Siberia can be considered an Ideal tank country. Without much natural defensive positions in the grassland and plataues past the Stanovoy range, Russia had stationed a large garrison there.
On the other hand, northern China is like the Ruhr to Germany, heavily industuralized and densely populated, given it's many cities and high value status, China had stationed most of her best troops there.
Russia will have a hard time invading and holding Northern China due to the immense garrison there with many defensible position and a hostile populance. Russia can do massive damage to the Chinese industial and economical potential by invading northern China.
China will have a hard time breaking through the Russian garrisons mainly due to the massive deployments of armor and anti armor platforms in eastern siberia. But if China gets through to the northern siberian plateau, Chinese armies can pretty much roll up to the back side of Moscow - without encountering much resistance or doing much damage to the Russian economy/industries as most of them were in western Russia.
So, the way I call it is, each side will not be dumb enough to start a war as their defensive positions are so strong.
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P.s. regarding the Navy, oh yes, navel intervention is a significant wild card. if Russia can convince China that she can land a raiding party anywhere on the Chinese coast will divert alot of Chinese material and personal to garrison costal cities - as the KMT raids in the 50s-60s have shown.
Alternatively, if the Chinese can convince the Russians that she can land a sizible force behind the russian defensive positions and cut off the one and only supply railway (just as the UN forces have done in north korea) - Russia will have to divert force into the reserves to guard against this and reduce the front line units facing the Chinese offensive.
p.p.s note that the Chinese and Russian railway have a break of guage too, logistics would inherently be delayed at border crossings -> tempting air strike targets don't you think?