I think that's a reasonable default scenario. Truth is entangled (and Draco/Harry/Hermione's dealings is more so than usual); I would expect that as soon as Lucius asked a question with an unusual answer, he'd keep asking questions until he figured out nearly all that Draco knew. If Lucius used Veritaserum and managed to not ask any such questions, then he might as well have not used Veritaserum at all...