I know you say the emergency brake is off,, but if it has been adjusted incorrectly,, or one of the cables has a restricted movement, it'll stop the emerg brake lever from returning enuff to relax the strut..
Take one of the return springs off,, and remove the solid cross strut, re-install the return spring, make sure the shoes are seated,, and retry the drum,, if it now fits,, then something in the emerg brake cables or the emerg cable adjuster,, is keeping the emerg brake from completely returning enuff to release..
