2007 IDrive here - Siri works perfectly fine over bluetooth. I can have a long convo/multiple lookup discussion with Siri, and it will only disconnect when Siri goes to sleep/disappears from my iP5 screen. Music will resume as soon as the siri convo ends, independent of input.
One note is that I do not plug my working phone into the y-adapter: I keep an older 3gs in my center console as an iPod. Perhaps plugging in your phone to the y-adapter is causing the audio/ipod function to override anything else.