In the Playbill.com article, it's clear that it wasn't Marissa's schedule keeping her from the project, contrary to what she's putting out in the press.
"About the genesis of the project, Burnett told Daily Variety, "It all fell together when I said, 'We've got to get Tracey.' A lot of times the networks were thinking about a teenager for the role, but that's not really what the story is about. . . . [Since much of the plot concerns the Queen's refusal to let anyone under her rule marry], it makes sense that the [characters] are more adult." "
Have I ever shown you my Shattered Dreams box? It's in my Disappointment Closet. - Marge Simpson