The session identifier is supported in CVP 11.6 and CVP will pass it to all the outgoing SIP messages, however to satisfy your requirement the Voice Browser need to handle the session id and pass it to VXML Server so that you could get it accessible ...

You can access the variable CallData.UUI in call studio to access the UUI, CVP passes the UUI to browser which then sends it to VXML Server.

I hope its not stopped because of barged in, few milliseconds chopping is what i have heard of but it seems you have big amount of portion of wav file getting chopped.Could you upload the relevant logs.

You can create a form element and then in the "Voice Grammar" property you can set "builtin:speech/transcribe".