My wife believes that the single point of failure was a known problem, and that what Artoo smuggled out of the Death Star was not simply the plans, but also the engineering and QA database and tech discussions. The rebels didn't have enough time to do the kind of full analysis that you'd expect would be necessary to turn up that problem on their own -- she thinks that the rebels just looked through the "closed" bug reports, and looked for something that would be exploitable.

So, the reactor needed a vent port, which makes a point of vulnerability. But it WAS ray-shielded, so someone clearly knew that it WAS a vulnerable point. And surface defenses were placed to make sure that no capital ship could get close enough to do a particle or torpedo attack.

She's sold me on this notion: for whatever reason, it's necessary for Star Wars Universe reactors to be able to vent to space. If this venting doesn't happen, the reactor blows up. If you shove stuff INTO the vent, the reactor blows up even WORSE, because it's like the reactor did the opposite of venting.

You can't particle shield the vent, because, if you did, it wouldn't be a vent any more. Whatever it is that the vent is venting would be blocked.

So they did whatever they could.

I don't really think of it as a design failure. I think it was an actual limitation of the technology, and that the Empire did the best they could.