I am familiar with Sutras (and Suttas), but not much with the Tantras. In what way are both different from each other?
1. Are Tantras exclusively like training manuals of various Yogic and ritual practices unlike Sutras which contain both practices as well as metaphysical, moral and general discussion of Buddhism?
No, tantras also present metaphysical, moral and general discussions of Dharma.
2. Tantras are meant to be secret where as Sutras are meant to be open?
Yes, because they also contain yogic methods and teachings which must be transmitted in certain specific ways since tantric practice is based on a very specific view of the human body and its role in liberation.
How do you Vajrayanists reconcile with this fact?
I guess if I had decided that the Western historiographical approach to Indo-Tibetan relgious history was definitive I would be a bit worried. As I have not, and am not likely to, I can read Indo-Tibetan relgious history as framed by western scholars with interest without it impinging my interest in the study and practice of Dzogchen.
Western academic studies of Hinduism and Indo-Tibetan Buddhism are often grotesque examples of colonial bias in action. The whole anxiety of Western and Westernized Buddhists (especially in Western neo-Theravada) about "Hinduism" quite frankly is a result of this colonial bias against Hinduism, just as your post betrays.