I ran into sort of same thing trying to pair my Garmin 665, phone, and radar. I listen to XM radio through gps, so need that to be stereo when I do. The phone I want that also to be stereo when I lsiten to Pandora. and then comes the radar. The radar is the most important thing to me, but to Sena it is the least important. So this is how I got around everything. When I want to listen to GPS, XM radio, I will pair to Sena as a stereo(AD2P) then I will either pair phone(mono) to gps or directly to Sena. When I want to listen to Pandora, which goes throiugh phone, I will pair phone in stereo and gps will pair as mono. To switch between channels there is always a slight delay, maybe 1-2 seconds(too long for me with radar) I want instant response with radar!!! I do not want a delay at all! So to solve that problem with me, I installed a small tiny speaker in my helmet and hard wired(coiled cord) in my radar. When I get on bike I just plug coil cord into my helmet and done! Yes, I have to plug in one wire, but it is worth it to get instant radar response. It may sound complicated with how to pair phone and gps but once you do it a couple of times, I actually can pair either one while riding with no issues. This may not work for others, but for me It works!!
I do love the Sena headset, wish I got it a long time ago!
One other thing I learned is that my other gps...Nuvi 765T can only pair in stereo, where the 665 will pair in either stereo or mono.