No contest. There's only 2 roads (A331, R258) and 1 railroad linking western and eastern Russia. While out of reach of China in the past now they're trivially in reach.I know there are many pro Russian people here, but isn't the obvious fourth example a colour revolution in Russia, making them an EU member? A new but slightly more competent Yeltsin provoking China and placing missiles near the northern border and supporting terrorists in Xinjiang, like the Soviet Union did before WW2. Border clashes escalate and China decides that if there is going to be a war, they might as well take back outer Manchuria. The PLA ground force could truly show their worth when fighting a traditional ground war against Soviet/Russian tanks in the north
Russia knows this too so even the stupidest, most pro EU president won't do it. It is actually key to Russia today to be at least neutral to China.
It helps that the pro west liberals in Russia have 0 popular support. If Putin falls #2 in the polls is CPRF.