For a very long time, a great deal of the Badlands was ruled by human tribes, who were vassals to Nehekhara. They'd have directed the flow South to Nehekhara - a much more logical way of drawing resources down than just tearing out the nexuses, when the whole point of these things is to be pulling magic in. Especially when, again, Nehekhara had its own version of the Waystone network in its pyramids.
A gyrocarriage's engines are extremely loud. This was not a stealth insertion. If anything was there to object, they'd have shown up when Mathilde had Adela fly one into the heart of the clearing. And burning a bigger hole would hardly be a problem with Adela there. A Nexus is, as described multiple times in the update, just a big rock really. A team if dwarves could have it hitched up securely in less than an hour - more than long enough when a Waaagh has pulled every goblin in earshot away to attack the beastmen.