Needing to be in range doesn't mean he has to be out in the open, talking, and/or drawing attention to himself. Honestly, they didn't even pay him any attention until he started talking, and he was just a heap of flesh until he regenerated, and even then he was still able to start controlling the others. So yeah, I don't consider him too smart.
He does, and they're not on flat terrain. You can check the previous chapter and see that there's giant mushroom-like things spread around, rocky ground and high edge cliffs, crystals all over the place. So there's plenty of hiding spots he could've used.