I loaded a save before re-entering Broken Valley, and could easily get to G3 and pull the lever, even without disabling the first two generators. Actually, landing near the northern of the two western anti-dragon zones, you can get to the teleporter without having to drop that shield, or simply bypass it and run to the southern section. If you don't use the teleporter a stretch of that path is within the poison, but nothing you shouldn't be able to handle at a high level or with a couple healing potions (I was level 31 and probably could have made it without healing, but used a potion near the end to be safe).
If you got to the two headquarters and the mine, there there is no problem not lowering that anti-dragon shield. You can clear everywhere within there in human form, and despite fact that the shield should have dropped, you don't actually need to be able to fly through there.