People always say Dumbledore would be way too stupid if he trusted Snape, but think about it - he would be more stupid to come up with a conspiracy like that. Weigh the pros against the cons of all the courses Dumbledore might take before you go bowing to the Gods of Deep. And for heaven's sake, remember that Dumbledore already had a plan in motion to save Draco that did not require Snape's involvement at all.