Your history's pretty funky. While you're correct about the whole idea of the Dark Ages Myth, the arguments you provide aren't the best.
It's true that not much was lost following the fall of Rome from a technological stand point, and the fall was more of a social and political setback.
However, it is untrue that no or little technological advancement was made in the Early Middle Ages. However, advancement was focused on practical instruments. Wheelbarrows, horse plows, the wind and water mill, etc. Technological advancement wasn't dead, it just changed focus.
Additionally, the tribes that conquered Rome were mostly
not nomadic. They were agrarian and were looking for a place to settle down, not to eventually move on with their flocks. They also were fascinated with Roman culture and tried to preserve it, particularly the Ostrogoths, but internal turmoil, and, in the case of the Ostrogoths, attacks by the Byzantines foiled those attempts.
It also had little to do with absolute monarchs; Rome itself was also an absolute monarchy. Meanwhile, the feudal monarchies that developed by the end of this period were most definately
not absolute.
Finally, the Church had nothing to do with stopping technological progress, in fact, monks were responsible for preserving much of Roman knowledge. Monasteries were the primary schools before the rise of the University system (which the church also contributed to).
And approximately none of this is relevant to Fallout.
Unlike the fall of Rome, the nuclear war fundamentally changed the environment and human society globally. Humanity wasn't sent back to the Middle Ages, it was sent back to the Stone Age... but now we had guns and man-eating monsters.
That's why you've seen so little advancement in 200 years.