Hi and thanks for the questions.
1) what is the protocol use between AMT swicthbox <--> remote console and AMT swtichbox <--> AMT-equipped PC(s)? Does AMT support SIP (session initiation protocol)?
Intel AMT Switchbox is a AMT proxy and so, makes use of the same protocol that you use from console to AMT directly. AMT does not support SIP.
2) Do we need dual BIOS support in case upgrade AMT firmware fail over the network? how this AMT firmware upgrade work?
I don't have an answer for you on this one.
3) What needed to be done if we need to enable AMT for IPv6?
Intel AMT has some support for IPv6. For example: Intel System Defence filters should also filter IPv6 without issue, and you should be able to access AMT's port 16992 to 16995 using the IPv6 interface if the computer has one. You can't setup an AMT computer in IPv6 static mode and you can't have an IPv6console subscribe for Intel AMT PET alerts. I say this from experience, not from any formal document.
(Intel AMT Blog)