1): Literally all of those characters. They’re mentioned in WC2 and 3, some mentioned more than others, but not an extensive background to them. They had the same effect Saurfang and Shen had. People liked them and they defined on them. Or they needed someone with authority to act, so they looked at the basically faceless NPC and decided to make him a functioning, acting character.
2: He doesn’t have to help the Forsaken, we’re talking about the Horde.
3: He met the Tauren (then Yongol at the time) long before he met Huln, though Huln is the example I went with. For the Horde as a whole, they fought during the invasion of Archimonde, the events of Hyjal (twice) and beyond. He does get a chance of forming a second opinion after the events following Grom’s death and the events of Hyjal, and he does. Even after the events of Val’sharah, as well. This is a trifling faction war that is beneath his Circle in the face of an even greater danger, I’m very doubtful that he’s going to march back to the requests of Malfurion and uproot his Circle to side with him, or order them to attack the Horde the root of all evil’s sword is plunged into the heart of the earth.
The only thing to really get their pants in a twist over is the burning of a world tree that wasn’t supposed to be there to begin with (dragon blessing or no, it’s still an insincere gesture.) and the blighting of Darkshore (which, while abhorrent, isn’t RILE THE WORLD AGAINST THIS ONE ARMY, razing fields and forests in times of war is common. It’s such a small thing in the grand scheme of what’s going on.).
4: Capriciousness, distance and being their own entity. They’re not all in contact with each other, nor aware of all events in the world. Some might not even care, because their domain is well and provided for. They are gods and fickle ones, save for perhaps Cenarius and a few others.
5: That’s part of the whole strategy. Even if the Gurubashi attack didn’t occur, the Bleeding Hallow’s attack would’ve not been so successful due to the defenses themselves. In a siege where you ‘have’ to overcome the defenses and not starve them out, the first wave (after being paid triple because they’re likely not coming back and need some encouragement) has the job of eroding the soldiers on the wall, then, if not all dead, rise gates and sabotage defenses. As you say, attacking a Keep isn’t a glorious battle, it’s erosion, usually by starvation but also by thinning ranks.
6: They aren’t the one and only Warlords of the Horde. The only problem with Horde writing is that they don’t make use of all those NPCs, despite having totally created them. They do exist and the Horde is in such a capacity that in the books, even Varian, a royal who was born and bred from youth to know about leadership in both the political and military forms, knew that, if he did dismantle the Horde leadership in SoO, it would’ve martyred them, united the Horde that was still on the fence following a coup and brought as much ruin to the Alliance as it would have to the Horde. In his gesture, he brought the war to a stand-still with words just dulled enough in their barb to both get his intended outcome to go through and still save face in front of his subjects.
7: The point I’m trying to make is that they’re not giving the Horde the deus ex ‘I’m totally vulnerable but I can’t die’ treatment, but rather providing story reasons as to ‘why’ the Alliance couldn’t have bowled over the Horde, because they’re not some helpless pile of cavemen who don’t know how to fight or do anything that means safeguarding their civilization.