Iraq is a relatively flat country. Did anyone remember how long it took for US to drive to Baghdad?
Do we know how much supply a million man army needs a day?
The approach for China would be to follow 'you fight your war, I fight mine approach'. Have the troops in Manchuria to avoid direct engagement with Korean troops, but harrass their supply line after their main force has passed. Mean while troops from beijing will try to held on to front line and reserves can be called in from the rest of China.
Again, it still takes good old infantry to hold ground. Did anyone remember how much troop Beijing was able to direct into Sichuan immediately after the 2008 earthquake?
Also, while the reserve number of the Koreas may look impressive, I serious doubt they had enough weapons for all of them. And are we expecting them to walk to places that need defending, pool cars or maybe use the bus? What's the strategic oil reserves inside Korea if wars cannot be bought to an end fast enough?
If SK is to be assigned against Beijing MR and NK against Shenyang MR, then the Shenyang MR should deny major engagements that NK seeks, and let them run out of steam. The NK then face the decision of further pursuing Shenyang MR or join SK in attacking beijing. If it was the latter, then Shenyang will harrass their lines while they are breaking off. Plus if you split NK and SK to face different MRs, then you essentially will only have SK forces pushing towards the chicken neck.
Second artillery missiles will not be attacking transport routes in China, but those in Korea.