1) It can be used as a remote Bluetooth speaker for the Amazon Echo Dot, but NOT the Echo itself to my knowledge, as the Echo only allows you to stream sound TO it, not FROM it to another device. The Dot does work however, and there have been articles suggesting using a Dot and a Tap together as a portable replacement for the Echo. There is one drawback, last I heard, the playback controls on the Tap will not work to control music coming from the Dot.
2) The Tap does not support AirPlay, but you can use it as a Bluetooth speaker from an Apple device in much the same way since both AirPlay and Bluetooth devices show up together on iPhones and Apple TVs.
3) Not to my knowledge, no. The one article I saw discussing multiple streaming suggested other bluetooth speakers (such as the UE Boom 2) that do support that feature if you need it.
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