Best Practices for Metal Roof Directly on Existing Shingled Roof?
Best practice is to remove the shingles and repair the deck as needed; I don't think anyone would consider leaving the shingles as more than "something you might get away with" - certainly not a best practice. 

From my "old-school" perspective, red rosin paper (moisture, not vapor, barrier, and not as slippery as tyvek, etc) and those furring strips you don't want, then metal. Depending on the current level of roof insulation, possibly also that foam you don't want.

If you have major concerns about rain noise, new shingles or a rubber roof (EPDM, white side up in your climate) might make more sense than metal.