Plot hole: There is no reason at all why, being targeted by a few arrows by unseen enemies - a fire suppressed already by the salvo of their own archers - the Rourans would turn around their heavy siege equipment, away from the bulk of the enemy forces, and fire it, hurling a single heavy stone to the middle of nowhere when they have the whole rest of the army who could storm the rock the supposed enemy commandos hide behind, or the archers who could keep shooting - again, they proved to be completely successful. It also makes no sense that the all-powerful witch who made the warriors flee managed to do any of this, 'sneaking' by horse in the middle of the steppe.
Suggested correction: Mulan used the helmets of the fallen warriors to make it appear that a large force has flanked Rourans. Rourans didn't expect this new "force" and knew nothing about it. They didn't know its size. And while their original target seemed harmless, this new "force" was killing Rourans. Fear and death were the reasons. What you see in this scene is an enactment of one of Sun Tzu's famous quotes: "All warfare is based on deception. [...] Attack him where he is unprepared, appear where you are not expected."
What we see in the scene is laughable, and not because of the idea, which surely is based on the profound strategic motto you mentioned and we find in many folkloric tales in other cultures as well; what we actually see in the movie, is that she grabbed a couple helmets lining them up on a rock, and she shot a few arrows. Then she stops shooting, and we see helmets knocked down in their full view. The movie truly surpassed itself in showing it in the most phony way; had they shown her shooting from behind the rock responding to their fire, or the helmets not falling, or them just shooting at mist, terrified, it would have maybe worked. It's an enormous overreaction. That and, under no circumstance trebuchets are used that way anyway. And she did all this setup unseen, again.
In response to death, nothing is an enormous overreaction. Something or someone was killing them. They wanted to kill it, and they didn't have time for Facebook's famous brand of pseudo-myth-busting. What if they knew it was one girl shooting at them? They'd still have done the same. Being killed is a very personal matter.
Plot hole: The 'avalanche scene' in this remake is mind-boggling. For starters, the Witch single-handedly holds the whole Chinese army at bay splitting in a zillion of flying creatures - a power level completely inconsistent with the rest of the movie. To the annoyance of being pounced by little birds, the army gets in turtle formation, apparently just waiting it out. The Rourans somehow are ready for this and have a trebuchet set up - despite the fact that they are nomads, conquered the forts infiltrating them, and they were skirmishing a moment before. They throw flaming boulders with such precision that they are able to target each single 'testudo', multiple times, with the soldiers just sitting there with no reaction.To save them, Mulan is able to sneak behind them UNSEEN, by horse, with a bunch of extra helmets she somehow carried, set them in place, and fool them. Any of these convoluted operations would have taken an impossibly long time.
Suggested correction: This entry does not mention any plot hole in all of this. All this entry does is explaining what happened in the film and then ridiculing it. For example, the Rourans weren't "somehow" prepared for it; it was their plan from the start. Being mind-boggling is not a mistake.
"Any of these convoluted operations would have taken an impossibly long time" is not a plot hole? They were not expecting a field battle (the scene literally starts with them saying "They left the garrison!", so were thinking of an entirely different fight), they somehow just happened to have those never-seen-before trebuchets in the middle of nowhere and have them ready for a usage that is out of their capabilities. I could have split the entry in a couple different ones, but the scene is the same and I think they provide adequate context to what happens with the chain of unpredictable and illogic (even in the 'magic' of the movie world) events.
I'll answer your first question: "'Any of these convoluted operations would have taken an impossibly long time' is not a plot hole?" In another mistake entry, you've complained that Rourans took their sweet time, and called it another mistake. So, according to yourself, no, it is not a plot hole. Clearly, you didn't like the movie and write just about anything to trash it.
Apples and oranges; you are comparing an inconsistency in the length/scale of a military campaign with the feasibility of operating a trebuchet, (inconsistent even in the same scene) as if the two could be related in any way. I could write a review if I wanted to simply 'trash' the movie, let's not try to attach motives when someone points out an inconsistency, it's not an attack to the movie per se, or to the viewers who liked it. Some things about this movie do genuinely puzzle me, sure.
Plot hole: At the beginning of the movie it is said explicitly by the Chancellor that the Rourans have attacked 6 garrisons at once, disrupting trade on the Silk Road, which would, in his words, threaten the survival of the whole Empire. It's a bit odd considering that their assault relies on the Witch's abilities, and she can't be everywhere at the same time, but forgetting that; the Emperor to counter this urgent menace (Bori Khan slaughters everyone in the cities) decides to summon to arms literally the whole kingdom amassing a huge army. This obviously is a project that takes months (we even see Mulan taking days just to get to the training spot, and then they train long enough to become proficient in archery when they started off not able to even throw an arrow) and does absolutely nothing to stop the brutal raiding and killing, but somehow Bori Khan's plan is kindly waiting on Mulan and her buddies to train, despite being a plan based on speed, surprise and distraction.
Suggested correction: First, it is not fair to cram so many "mistakes" into one entry. Second, it is your personal assumption that all six attacks relied on the witch. Third, she can transform into a bird and fly; certainly, she can catch up with multiple attack forces if needed. Fourth, it was an empire, not a kingdom; a super-massive empire called China. Fifth, training a relief force is also part of the call to arms. Where there is a battle, there is death. Dead units need to be replaced. Sixth, "speed" didn't come into play in the strategic aspects of warfare until World War II. At the time of this film, they were tactical elements. Wars went on for years, sometimes decades. China was a huge empire and conquering it quickly is impossible. Seventh, you've already explained the reason behind Rourans' delay into another mistake entry you posted: They were carrying catapults and they had practiced using them.
Entry is articulate because they are not separate mistakes, it's just that the "strategy" employed by the invading army and the response to it is all over the place and contradictory (1). They show and say in every possible way that the reason garrisons fall so quickly is because of the witch intervention and they depend on her (2). Catapults are never shown as being used for city assault (7), and it's obvious why; walls are bypassed, cities don't take months and huge armies to be taken, they fall in minutes (6). The climax of the movie itself happens with the invading army crushed, the Emperor knowing it, but their plan is perfectly successful, since they made it through the super-massive kingdom from the Silk Road battle, without being able to fly, simply outmaneuvering everyone with a tiny group of jedis (3-4-6 again). See original entry for why 5 is absurd;anything else I mentioned was not flavour or additional mistakes, but just context.
