But I'm not sure that he could really think highly of Umbridge from the very start. And Draco does not strike me as someone who can conceal his dislike and act in a politic manner.

I know this is old, but isn't that what stopped her bringing any of the IS along to see 'the weapon' at the end of OotP? She didn't trust them, him in specific, because he looks 'eager' when Hermione suggests the IS use the weapon on Umbridge.