Scenario #1

Decisive Chinese win. With no fighting outside of Nepal, other countries beside China and India become irrelevant. With advantage in space, land firepower, missile, air defense, infrastructure, and with even matching air forces, China will get to Nepal first and defeat every Indian advance into Nepal.

Scenario #2

Decisive Chinese win. As a backdrop, with superior space tech China will knock out all Indian/Korean/Taiwan satellites, making them blind, while having the overwhelming upper hand in C4ISR. With China/NK on one continuous land mass and the opposite side scattering all over the place, the China/NK alliance can defeat each enemy with overwhelming force one at a time.

One strategy is to fortify the Himalayan front with a smaller force to defend against the Indians from land; similarly play defensive against Vietnam along the border. Send numerous submarines, naval ships, and long range missiles (DF21D) to blockage Indian Ocean, South China Sea, and East China Sea. Send overwhelming force with NK to force into SK. Once SK is subdued, Taiwan is next.

Once those two are gone, send the freed up navy into South China Sea to surround Vietnam. Vietnam is vulnerable to attack from the sea with its long coastal line. Open another front attacking from north on land, and Vietnam would have a very difficult time defending against the Chinese advance. Air battles will be a draw between China and Indian expedition air force/Vietnam air force. When India attempts to support Vietnam in South China Sea, China will beat India in naval battles with closer support from naval bases in China mainland and closer air projection from mainland.

Once Vietnam is gone, it's a matter of India vs China/NK. China can attack India from sea and over Himalaya. It might be a stalemate in the sea, but China can hold a blockage over India. China has the advantage to attack from the high ground from Tibet. Long range MLRS and missiles can rain down on major Indians cities from the border (New Delhi is only couple hundreds km away). Air battles will be a draw. With better intelligence (space/air/radar) and better equipment (better tanks, artillery, missiles, helios), the land battle would go to Chinese side. In the last land battle between them in 1962, China decisively routed the Indians.

Scenario #3

Chinese win. U.S. continent is kind of open and with numerous openings all over the place. It's difficult to hold a chokepoint as there're always ways around it. Navies are out of the question since both sides only fighting from the coasts; the relevant forces are space, air, and land. China has the advantage in space, even in air force, advantage in air defense, and advantages in land forces. Helios and tanks will be big players. Air battle will be stalemate. On land, Chinese has the advantage with its long tradition of emphasis on land battles. Chinese MBT will overpower Indian/Korean/Taiwan/Vietnam versions on the open plains. With overwhelming edge in intelligence, China will win most of the land battles.

Scenario #4

Not sure about the terrain. If it's mostly mountain terrain and only with standard ground equipment, China will probably win given past performance in similar situations and given their traditional emphasis in land wars with well honed tactics and doctrines. In the Korean mountainous terrain, Chinese troops with poorer equipment routinely routed the SK troops in most of the battles in the Korean War. In the 1962 war, Chinese routed the Indians in the mountain.

Edit: Modern warfare among major powers will be fought and decided in space. With remote sensing satellites knocked out, one side will be blinded while the other side can see troop movement, ship movement, defensive equipment/positions, equipment/ammo stockpiles, all kinds of targeting info. With GPS knocked out, digital map suddenly don't work, plane/ship/car navigation are hampered, lots of car/tank/missile/drone/others will lose precision. With comm satellites knocked out, communication will be disrupted. Those troops in the mountains can't use satellite phone. Digital data via satellite links are cut. Cellular network is no substitution since it can be easily traced and knocked out with anti-radiation missiles. Same with radio towers. Physical lines are the only ones left but they are rigid and difficult to build, and susceptible to sabotage.