För att posta @Mithrandirs kommentar som svar:
Finding someone who is disgruntled - they'll seek out the Order by themselves, like Snape, or killed, like Regulus. Imperius Curse - risky. People can throw it off, like Harry, or it can wear off, and then they'll be in even more danger, being then their attempted spy could tell the Death Eaters about the Order.
Det finns starka bevis på att Malfoysna försökte komma tillbaka i Voldemorts goda grader, så kanske de inte skulle ha vridit på Voldemort trots allt.
However, Draco refuses to say that it's Harry when the Trio was captured and brought to Malfoy Manner, and Draco's mother lies to Voldemort about Harry being dead in exchange for Harry tipping her off about Draco was still being alive. Those aren't necessarily evidence that they would've flipped on Voldemort, though.