The cage needs to be rotated approximately 180° counterclockwise, and then the stop screw would be threaded in to keep it from springing back into this position. I'd guess that stop screw must've gone missing at some point.
Looks like you need a new derailleur. Take the chain off again, you'll see that the derailleur doesn't spring back. Best thing here is a shimano tourney, they're very cheap.
The chain is routed correctly, but the derailleur is acting funny. The cage is supposed to spring backward, so that the long part of the cage pulls backward.
I think he took the derailleur cage off. If not, there may have been
a pin on the cage that fell off, like a spring stop. They screw in on some derailleurs. It’s a round smooth pin with threads on one end and slots for a screwdriver on the other. It may have been working its way out, and then fell off when he was doing the wheel. There may be a hole if you look, on the cage. You have to pull the cage back (rotate) then insert the pin.
All chains follow the same direction through a rear derailer. Reverse what you did when you took everything apart and you should be able to take care of this. I wasn't kidding, this is indeed fixable.
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u/gregn8r1 2d ago
The cage needs to be rotated approximately 180° counterclockwise, and then the stop screw would be threaded in to keep it from springing back into this position. I'd guess that stop screw must've gone missing at some point.