There are fewer than 300 US troops in the entire DMZ and it's been that way for years. the large majority of US troops are not in Yongson (in Seoul) but much further south and even the size of the post at Yongson is being reduced.
Roughly 35% of all South Koreans (men, women and children) live within range of NK guns. The majority of US troops are further south.
The trip-wire theory (US troops at the first line) you cling to is decades old and long abandoned.
There are some anti-American young in South Korea but no more than there are here. They get the cameras spinning when they make noise. The purpose, as always, is to create the impression that the noisy few are the majority. Did they fool you? Looks like it.
"From what I've seen recently, the South Koreans would welcome North Koreans as their cousins and turn on the US."
What did you see, where did you see it? Your vision means that the ROKs would welcome a giant step backwards. And btw...they are cousins.