Ned Stark didn't die heroically. Nor did those at the Red Wedding. Nor did the Prince of Dorne. They all died horrible deaths. That is what I've come to expect from this series
All the "senseless" character deaths were major pivot points in the story line. Any beloved character dying in the major battle needed to also advance the story line (which is almost over); or either a heroic death, or a funeral sequence, so we could get closure. Anything else would have been bad writing.
Cavalry, historically, doesn't charge into massed lines of infantry head-on. They are used as screening forces to hide troop movements, scouts to gather intel, flanking or rear attacks due to mobility and to mob up a retreating enemy
Name one time the Dothraki ever lost a battle. They charged into the "men in their metal suits" and concluded "They do not know how to fight". The Dothraki were invincible on horseback and would have been useless inside the castle walls. They used a tried and true strategy. They (and we) were stunned when the Dothraki lost so quickly. If you were not moved by the extinguishing lights, then you have room to complain. But I think as a story point, and the result it was perfect.
The Dothraki aren't a heavy cavalry force though. They're an extremely mobile, unarmored cavalry, similar to the Mongols. They aren't carrying lances, but use curved blades (which aren't good at cutting through armor or bone).
Again, they mowed through the Kings Guard. Armored men, in formation, and the best fighters in the country. Jamie commented that it was the most astonishing thing he had ever seen.
The biggest danger is having their own forces bunched up if the undead dragon comes a blasting, but they have the dragon advantage in the battle and should be able to keep it occupied and off their troops.
The dragons took on the army of the dead and lost one 1/3 their force. If they attempted to attack, in low visibility, the Night King would have easily killed the other two, especially with the NK now also having a dragon.
In fact the show runners pointed that out. Jon tried to stop Dany from taking the dragons up so soon. When Dany went anyway, Jon had to choose to stick with the plan or to stay by her side. Aunt or no, he still loves her.
As for "Bran knew" comments... Bran can see all of the past, though he has to look for it. He can also see now, by worging into creatures. I may have missed it, but I have seen nothing that concludes he sees the future.
As for the Night King... I don't know if his goal was ever conquest of Westeros or the world. The working theory at the firehouse this morning is that the NK and the 3-Eyed Raven are connected. 3-ER (who moves from living body to living body) and the NK go back to the time of the first people. Perhaps 3ER is responsible for creating the NK, and now the NK, who has been cursed to eternal unlife only wants a force to be amassed so he can stare 3ER in the eyes and finish him off for good - or at least be killed himself. That would mean the NK strategy of stepping in the front lines was sound - at least from his point of view.
Unfortunately, that theory cannot be verified until the prequel, if ever.
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