Although I don't know the answer to most of your questions, I surely know the answer to the first: [NatCam][1]. It is a fully cross-platform camera solution specialized for iOS and Android. It is blazing fast, has a ton of normal camera controls (like flash, focus, zoom, torch, and so on) and has amazing support.
[1]: https://www.assetstore.unity3d.com/en/#!/content/52154
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