First of all, I must give you credit for explain your problem and the things you have already tried properly and with all necessary details. People often drop a line and two and it takes lot of time and people to dig down the root of it and gather required info. So, thank you!!
Now, lets check the problem, I have one small question, which is silly as well. With Cp 6, as you know we have two installer for WIN 32 bit and 64 bit, I see from logs that you were installing 64 bit of Cp6 Voices, I hope you have the 64 Bit version of Cp as well.
You can find both installers from here --- https://www.adobe.com/cfusion/tdrc/index.cfm?product=captivate
If you open the VT folder, do you see the Name of speech agents from CP TTS or not?
I checked the logs, I can recommend one KB for the error code given there : http://helpx.adobe.com/creative-suite/kb/error-base-product-installed- installing.html
Hope this gives you success..
Cheers,
Anjaneai