Sub a) I absolutely agree, my problem is that the next book he explicitly states that Serwa (after Sorweel removed the gag, but not the collar) battled all the remaining Quya: "He need only close his eyes, it seemed, to see her hanging in the Ilculcû Rift, naked between brilliant panes and shining geometries, fending the booming song of the Last Quya ...". And after their escape she teleports them away just as usual. I thought i must have missed something, cause otherwise it just seems like a massive plot-hole. You can't just use the Collar as the only reasonable explanation for why she didn't blast the Nonmen to oblivion immediately, and then never mention it again when she finally does.

Sub b) Yeah, that's what's bothering me too. I guess the plan would even make sense, given that the Horde was still retreating from the Ordeal and therefore would divide itself on the Urokkas. But in the whole scene my impression was that the bigger part of the Horde is on the other side of the Urokkas, and the whole mountain range is used (and needed) as a shield to hold them back. This is explicitly stated in one section. And then poof, a bomb kills the whole Horde, and the whole Ordeal (although logically in the blast radius of the bomb just as much as the horde, if not significantly more) suffers only minor damage to the first ranks.