To reenter: 1. the Fairy must be living with you, 2. you need to talk to the Fairy and she needs to tell you to bring her 10 watermelons, 3. the owner of the magic shop has gained weight befor you were to big to enter;
I think that should be all.
I have completed the game in terms of weight, currently at 500 kgs(got the fat streamer ending and normal shower scene ending with Jamie).
I am also unable to enter the magical shop due to my weight. People in the comments below say to talk to the fairy. But the only options I get after I click on talk to fairy are the apple, cantaloupe, watermelon. No such things as 10 watermelons or anything that can help me pass through. I have also not been able to do any gothic events.