I think you are definitely overthinking this. I don’t think they are worried about wet signatures. If they have a question, they will call your relative in Ghana to confirm. When my wife was there (DC), they asked her to sign for her dad when she left the signature part blank. The important thing to them is that the form is filled. I think I was my wife’s other guarantor, and I think I just put my US passport number in and crossed out “Ghanaian” on the printed form.