Found a way to get down the hellmouth solo without building the bridge. Once you reach The Stills you can't really go anywhere because of turnback barriers, ̶b̶u̶t̶ ̶I̶ ̶g̶u̶e̶s̶s̶ ̶t̶h̶i̶s̶ ̶c̶o̶u̶l̶d̶ ̶b̶e̶ ̶u̶s̶e̶d̶ ̶t̶o̶ ̶g̶e̶t̶ ̶a̶ ̶b̶o̶n̶u̶s̶ ̶c̶h̶e̶s̶t̶ ̶i̶f̶ ̶y̶o̶u̶ ̶a̶r̶e̶ ̶l̶u̶c̶k̶y̶ ̶w̶i̶t̶h̶ ̶t̶h̶e̶ ̶c̶h̶e̶s̶t̶ ̶s̶p̶a̶w̶n̶ ̶a̶n̶d̶ ̶y̶o̶u̶ ̶c̶a̶n̶ ̶s̶h̶a̶t̶t̶e̶r̶s̶k̶a̶t̶e̶/̶w̶e̶l̶l̶s̶k̶a̶t̶e̶ ̶f̶a̶s̶t̶ ̶e̶n̶o̶u̶g̶h̶ (EDIT: Apparently you cannot get the secret chest using this OOB :/)