Without a treaty, your first scenario would be a outright "interference of another country's domestic affair" or "regime change". I don't think the west would be welcoming Chinese intervention, rather they will pick the chance to demand China to hand over NK for a "free election".
Without a treaty, you expect SK to welcome Chinese occupation? That is like asking China to do the pacification job of NK and then what? Hand over NK to SK for free? Making NK people to hate Chinese as much as they hate USA? I don't think Chinese are that dumb. I think I had made it clear that China is not going to pay for that, SK has to do it themselves.
I think we have a big gap that you have a very high favorite or trust to SK which I don't have.
If someone in NK invites Chinese troops in, they will be there at the invitation of someone in NK. That doesn't count as unwanted interference.
Would the US welcome Chinese troops in NK? but what could they do? Unless the US was willing to go to war with China, they will have to accept the presence of Chinese troops.
We saw the same thing happen in 1998 when Russian peacekeeping troops occupied Pristina airport in Kosovo, and NATO troops refused to go to war with Russia and start the escalation chain to nuclear war. Then a political settlement was agreed.
I don't think SK will welcome the presence of Chinese troops in NK, but again, what can they do?
They can go to war with China, or accept a political settlement that sees the end of the US-SK security alliance, and which would mean a neutral SK with closer relations with China. Then everyone in NK, SK and China get on with the job of rebuilding. Remember that SK lacks the capacity to do this by themselves, but China does have ample spare capacity, if it chooses to do so. China will certainly not be handing over NK to SK for free.
And I've previously outlined all sorts of things that China could do, if US troops remain on the Korean peninsula.