Just because it works some of the time (or even all the time for you) doesn't mean it's implemented correctly. While we can only speculate on what kind of disaster the current implementation is, it's pretty evident that error handling is pretty much nonexistent and that multitasking is very poorly implemented. The reason the nav is likely screwing up when bluetooth music streaming is running is because it can't handle both tasks at once.BT streaming usually works with NAV for me
An experiment you can try, if you have an Android phone anyway, try installing a bunch of applications while streaming music (if the phone lags and the music stutters, that's what you're trying accomplish) over bluetooth, while using navigation. See what happens.
Most of the effort involved in creating software is error handling. Errors happen all the time and don't necessarily mean something is wrong with the system.
If the information I have gotten is accurate, I do expect the new version of MFT to be well-written software. But, we won't know until it's in our hands...