Really easy fix is to have Obi Wan sense Luke is in danger while he's fighting Vader. Gives him a valid reason to get the **** out posthaste instead of meandering away like he did leaving Vader to terrorize the galaxy. Have all the post fight exposition during it instead so Obi Wan has even more of a reason to go ultra aggressive during the latter stages of the fight.
This shit really isn't that hard to do and it's baffling Disney keeps fumbling the writing like this.
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