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1: only between waves of the same type. A bit of electric field (light) can't cancel or reinforce a bit of pressure (sound)
2: there are lots of possible answers. diffraction is when a wave changes direction due to encountering an edge. Interference is when two waves encounter each other and reinforce or cancel one another.
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1. Well, there has to be some coupling between the underlying forces, but yes. What you are asking is essentially whether sound waves can be made visible, and the answer is yes, because a sound wave changes the density of the medium through which it moves, and this affects the index of refraction, so light waves will also be affected.
2. Interference is a general term, for what happens when two waves meet. Diffraction is a particular type of light behaviour. It's possible to consider diffraction a consequence of interference, if you take the Huygen's Principle approach to describing it.