Economic collapse would trigger a massive humanitarian crisis and China would be left to deal with the refugees as they take the only viable option and migrate north into China.

I don't see how this is unavoidable and China has some responsibility in that seemingly inevitable result, barring turning the peninsula into glass.

Why can’t Washington soften a bit and show a willingness to talk without pre-conditions? What does it have to lose?

President Trump knows how to negotiate... I definitely have confidence in that.

The 5d chess on this peninsula is challenging. Success would look like de-escalation of hostilities without nuclear missiles being launched in or out; peaceful handover of power to someone capable of stopping a counter-coup without economic collapse; peacefully restore human rights to all North Koreans imprisoned in work camps

If you can't accomplish all three of those near-simultaneous, the remaining will more likely happen on an extended timeline. South Korea and Japan are facing nuclear disaster. China is facing refugee floods when they have enough population problems. North Korea is besieged with weapons they can't defend against (even non-nuclear, Rod of God for instance is most likely available)