So why would any leader trust the USA not to interfere?
I'm sure he doesn't. He's stuck between a rock and a hard place. China also interferes in a "pro-market" counter-revolutionary manner which threatened KJU's absolute rule (and probably his life). He's run out of friends - Trump's given him a life-line. Not saying that's a bad thing, but KJU's gamble is possibly that he'll have full support of China if he can get the US out of South Korea, lose his nukes, but still be under China's nuclear umbrella. He'll also probably get a gas/oil pipeline from Russia, extract royalties for shipments to South Korea, improve the NK economy if all goes well, but remain as dictator.