The deniability doesn't diminish at all. You can even testify in court that the drive is encrypted using TrueCrypt and that TrueCrypt has plausible deniability. The whole point of TrueCrypt's plausible deniability is even when you know about the feature, you simply can't mathematically prove whether or not it's being used.
not directly, but their faq pretty clear about a bunch of secondary ( out of band, internet updates duplicated, etc) correlations that could lead to good evidence that the second system exists. pulling that off is noeasy task.
