No you do not, however it would make it easier. To use multitrigger i suggest you take a look at andys wifi script. simply let all your yi-s connect to your wifi router, or conf one of your wifi cards as AP, and let the cams connect to it.
There is config file for multitrigger (called...
i didnt try the power pins :p just the AV out feature
its not like xiaoyi is gonna develop their own flavor of usb pinout :D
you can do both.. just to clarify, there should be power adapter between wall socket and the camera ;)
Wifi speeds have been improved in 1.0.12 firmware
Multiple camera streaming - possible, not easy.
USB streaming, that would need quite alot work..
HDMI - you can do that by default