Entering the shed: it seems like there are two triggers. First, you have to
have made it into the jewelry shop (I forget whether you need to have gotten the crucial object inside though)
Then, you need to
tell the sorceress that you have met the person inside, but it doesn’t have to be in the same lifetime you’re on now (but never use >RESTART, because that will erase all the triggers). It might be hard to know how to steer the conversation that way, but save/restore if necessary before starting to talk to her and the option will pop up eventually I’m sure
If you’re stuck on accomplishing the first trigger,
you get in the same way the sorceress lets you in the first time you visit her
Do those things and the shed will just be open (and the room desc will say so) - nothing special you have to do to get in.