Its so nice to hear from you and that yet another branch of the Schetelig family is interested in the family history, sadly that is the same info I have, nothing on Andreas Shedlich life or ancestry or even about his wife. I'm also still looking for info on Dorothea Sophia Petersen's family, I've found nothing.
What I do have, but I'm sure you do too, even though you didn't write it down in your reply, is Hinrich Müller's parents, Hinrich Friederich Müller and Margaretha Petersen. I don't know if its the same Petersen family, but it might be a clue to how Hinrich and Margaretha Dorothea met, they might have been cousins to some degree.
Here you can find a lot of info on Hinrich Müller (p.271) http://www.rosekamp.dk/DBL_All/DBL_16_text.pdf
I am a descendant of Johann Friedrich (1762-1817), Margaretha Dorothea's brother.