The ministry are idiots--Sirius went to Azkaban without a trial, for instance. (And furthermore, wouldn't it sew more discord if you were suspicious of everyone, even people who didn't have dark marks? It'd fit Voldemort "his gift for sewing discord is very great").
Yes, they are idiots. But Voldemort is too.
Now that I think about it, that's actually not possible--Dumbledore is not a Parselmouth--he can't open the chamber.
Actually, Dumbledore does speak Pareltonuge. At least enough. More than Ron, who also gets into the chamber of secrets pretty easily. He demonstrates the ability in the memory with Bob Ogden.
I...don't follow the logic; why would being a former death eater make Snape a better candidate to tail Quirrell? I read it more as "Dumbledore trusts Snape more completely due to love."
He knows how Dark Wizards think. And hell who knows, it might've been a test for Snape. I mean, Dumbledore could've handled it all himself. He's capable of becoming invisible without a cloak, and let's face it... he'd really be the best choice to keep an eye on him. Dumbledore does love his tests, what with testing Harry with it and all. Instead of directly dealing with Quirell, he left it to Snape and Harry.
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See You In Another Life, Brother.
New York Giants - The Patriot Slayers