I don't know if it's possible to develop atomic science without stumbling into high-enthalpy chemistry along the way. Much how I don't see post-collapse nuclear waste safeguarding as that big of an issue: if we bury it deep enough in a shaft, and ensure the shaft collapses and fills in after a certain period of disuse, then anyone who can dig that deep will need modern excavating technology, and they'll probably have figured out what radioactivity is along the way.

I'm not sure what the maximum range of any trebuchet is. Probably has to do with wood strength and the object's terminal velocity.