True, but:
* given the US's nuclear submarine, bomber and ICBM capability this was likely seen as a moot point. The threat was still there. Indeed, one of the reasons the US cited for removing its nukes was that they were no longer needed.
* They actually never fully confirmed this withdrawal until 1998.
>Overall, my impression is that North Korea fears a conventional ROK/US invasion much more than a nuclear exchange scenario
I think so too - for now, but it probably didn't seem that way in 1958 when they had nukes pointed at them.
>I also don't think the situation would be different if NK didn't have a nuclear arms program
I do. They'd be much more vulnerable to invasion. Given the US's military history in the last half century (Vietnam, Iraq, etc.), I think their fear of that is somewhat justified.
Their alliance with China does provide some protection but relations have soured over the last few decades and if I were them I wouldn't see it as a relationship they could depend upon.