You make an excellent point about the dark mark, and I just thought I'd add this to support the "no Dark Mark" theory:
During the final battle Voldemort has called the death eaters to his side in the forbidden forest, but Draco is able to remain in the castle. If Voldemort summoned all Death Eaters to his side --Draco's mark would have burned, and this suggests that either he didn't have one, or-- he definitely defected.