Once all the arms are connected, go back to the main ruins, and go down the path to the stone globe with the seed-swap device.
Use the device to teleport back to Hunrath.
Make a note of where you appear. Run to where you left your blue beam cart, and bring that cart back to neighbouring track on the other side of the wall. While still in the cart, reach through the hole in the wall and throw the seed-swap device switch to teleport back, along with the cart.
Now you should be in a small area between two wooden walls. Back up a little so that you can aim the blue beam at the Mofang laser (through the window), which is the red beam. (You should be looking towards the left - if the window is to your right, back the cart up so that you're nearly blocking view of the Mofang device with the laser aimed fully right.You can just barely hit the device. Teleport back, turn the cart around by driving around the circle in the junkyard, and return.) Once it is destroyed, teleport back (you can reach the seed-swap device from the cart). Now make your way back to the point overlooking the junkyard (move the cart somewhere out of the way) by following the path once again (you can also simply take a shortcut using the purple portal and rock path you took to get here in the first place). Teleport back to Kaptar with the seed-swap device.
Throw the lever to reposition the bridge, and go to the center, moving it again. Cross the bridge (away from the seed), turn around and push down the vertical silver stopper that is blocking the bridge from turning further. Go back to the bridge centre and turn it in the same direction again one more time.
Walk back in the seed direction (but not quite) and you'll pass through a brown portal. Climb up the stairs, go onto the platform attached to the chain, throw the lever, and ride it across.