Yeah he and Edith will continue the facade that Marigold is their "ward", but I heard that adoption was made legal in 1926, so maybe they can go to the registrar and sign their names as well as Marigold's to strengthen the story of them adopting her. Marigold was an orphaned daughter of a friend, Michael Gregson but he died and no one knows what happened to her mother, and Bertie may put his surname Pelham to show that he and Edith are Marigold's foster parents and are willing to take as a "surrogate daughter" to hide any aspects of her illegitimacy I think then Bertie and Edith will be protected by scandal in some way. But I believe that Marigold will never know the truth about her father until she attains a great age, probably 20-30, but I think Marigold knows that Edith is her mother cause Edith always showed affection to her and always visited her when she was at the Drewe's so yeah, its just her illegitimacy and her late father, whom she did not know of and nor did Michael know about Marigold so yeah she will be told way later.