I'm not going to be able to address all your concerns but I had the 150 for a while and found The bluetooth connection issues were awful right after release but after getting firmware updates they were pretty darn stable.
As you have already noticed, Bluetooth connections are finicky across all devices. In general, ALL things on all devices with Bluetooth might "go to sleep" after a given point in time if they are not actively being used but they wake up (reconnect) when you try to use them. I believe this is a power saving feature but it does cause a noticeable delay before you get the response you are expecting.
The "guitar in" won't be stereo. It's probably Tip-Sleeve only for guitar. I don't recommend running music in that way.
The Aux in on the back panel (I'm pretty sure) is the 1/8th inch Tip-Ring-Sleeve jack you would use to connect your MP3 player/phone/external player to the 150. You should only need a 1/8 to 1/8 male cable. I think you have to push down on the main volume control to adjust the volume for the aux in jack. The color circle should be a different color for AUX than the color ring for the volume control.
I agree with your very last sentence about acceptable quality. I believe Bluetooth was originally intended only for wireless headsets to be used with phones, not music quality. In my opinion the technology got hijacked for music because it was accessible and because it was believed average folks wouldn't notice the quality issues.
Take what you need from my rambling and leave the rest behind :-)
