Chinese troops got involved because US and its allies forces counter offensived and pushed back the NK forces in South Korea and went deep into NK.
I don't think NK soldiers are cowards. Maybe some but not all. Some are very loyal and fierce. Chinese technical folks can and go into NK to assist them,
The baseline is NK is not worth enough to damage the economy if Chinese and US troops butt heads directly.
Yet, it worths enough to give some help to NK to fend off US attack indirectly.
US strategy is to choke the oil supply to NK just like it did to Japan in WWII.
NK is not going to wait till that point when it's tanks and military trucks cannot run.
It's going to do something.
Going against the US directly, you go all out or you stay home. North Korea cannot defeat the US with any kind of assistance unless you count fighting a war for them as assistance. If you supply them with weapons, train them in basic operations 101, they will lose, Chinese weaponry will look like garbage, US will dissect the systems they capture and everyone will know China tried to help North Korea but failed and the US prevailed. This is worse than just watching them get their asses handed to them. If the US is to be defeated, Chinese troops will have to be deployed just like in 1950. If China decides it doesn't wanna butt heads with the US economically, then the best way to preserve control of North Korean territory is to destroy Kim before the US gets there and take over North Korea.
The oil choke may be America's strategy but China and Russia can supply oil to North Korea via their pipelines to keep it barely running. North Korea may
want to do something but it has no options other than cursing out Trump and calling him a dotard. Any attack that Kim launches will result in his immediate destruction; it's absolute suicide. If that happens, China will be left with no defensible excuse for helping North Korea and will have no choice but to destroy Kim's regime and take over its territory before the US can.