This is awesome — great work. I have always wanted to have my Echo integrate with my other home audio equipment.
You may be able to solve your volume problem by putting in a line out converter between your Echo and your Sonos Connect. I have used a Scosche converter (http://amzn.com/B00009UHRE) for car audio that knocks out the amplification in order to connect the line to an amplifier. I have not thought about doing it for home audio, but something similar just might work to solve your volume problems.