Rahvin had a line of retreat planned, and he had defenses. IIRC, RJ has definitively explained that he simply wasn't expecting Rand to jump right into his throne room. I think in most situations, that would be one of those things that would be so stupid to try, that actually doing it caught him by surprise. And stuff like that happens all the time. To borrow a couple of examples from World War Two, the German invasion of Norway and the Japanese capture of Singapore. In the case of the former, it would have been a colossally stupid move for a nation with almost no navy to attempt a naval attack, against a country that was closer to Britain than to Germany, across the North Sea, which was for all intents and purposes a British lake, against the greatest naval power in the world, and certainly in Europe. But the Germans caught the British by surprise and did it anyway. Likewise for the British fortifications as Singapore, which were so powerful they were convinced it could not fall, except for the fact that they did not cover the jungle to their rear, because that was impassable. So the Japanese went through the jungle anyway, and took the British in the rear, when the slightest extra defense would have rendered their attack futile.

If entire staffs and committees can routinely make blunders like that, it is not unrealistic or "plot-mandated stupidity" that a single guy, trying to handle everything himself would make similar blunders.

Also, it is important to remember Forsaken priorities and mindsets. The Forsaken see their archenemies and greatest threats as one another. Rand is just an inconvenient local without a fraction of their talents or knowledge. And that's not even really underestimating him, because they are not interested in winning the fight for the sake of their cause or the Dark One, they are all about winning the Dark One's favor, so as to be elevated above all others by their master. As far as that goes, Rahvin (and Sammael & Graendal, and even Bel'al had he lived) in particular, would see furthering the reign of the Shadow across a major kingdom as more important than whacking some unschooled irritant, who really had no chance against the Great Lord anyway. So with that in mind, their defenses would have been prepared to stop Forsaken, not so much Rand. Forsaken would not have risked their own skins by jumping blindly into an unknown setting to directly confront an enemy on his own turf. Hell, even Rand isn't dumb enough to do that. Rand, who earlier lost a companion due to his inability to react decisively to a Forsaken threat, who was already pissed and upset about how his indifference cost his future children a grandmother, and who had just seen one of his best friends and one of his girlfriends get killed in the street ... that guy is a whole other ball of wax, and something that most of the Forsaken could not conceive of, as the necessary degrees of affection and selfless interest in other people, or right and wrong, are way out of their wheelhouses by this point in the game.

Graendal and Sammael touch on a lot of this mentality in the next book, where even the one who is the strongest advocate for cooperation against the threat of Rand is unwilling to extend the necessary trust, and where she comments on Sammael's inability to grasp the effect of sentimental values on Rand's mentality. Even her own categorization of his personal motivation is somewhat short of the magnitude of the effect the cause she cites is actually having on him. They just don't get normal human motivations, and they are more concerned with the threat each other poses than Rand.

Rahvin had an adequate plan, and he did not utilize all the resources that we would consider to be available to him, because in his eyes, they are not. Trusting Graendal or Sammael to guard his back from the threat of Rand would be suicidal folly from his perspective. Trusting them to support him on a battlefield against Rand equally so. Stipulating what I said about each Forsaken being more interested in winnowing out the competition for the Great Lord's favor, than in eliminating a nuisance, and applying to all of them, a Forsaken who finds himself present when Rahvin is going head-to-head against a determined, if ignorant channeler of Forsaken-level strength, is far more likely to seize the opportunity to take a rival off the board. And each one of them knows that, and will never-ever let himself be put in that situation. That's why Sammael was demanding to part of the link when Rand comes for him according to their plan - to prevent the others from doing exactly that.