If you have bare shingles under metal roof panels with expansion and contraction with wind load it will rub scars into the panel itself. Imagine taking your roof panels and pushing them across the asphalt on the street. That’s what will happen over time making it bare metal to rust. Batons are like a spacer between the shingles and the panels.
no so you actually use 1x4 have to have one on the edge then 1 foot from there then you have one 2 foot from your one foot then each board after will be 2 foot. Gives it more structure so the panel doesn’t collapse on itself especially if your using thin metal like 26 gauge steel. Board spacing can be different in different areas with the wind loads. You do not need to cover the shingles if you run batons for metal. But you will have to take the bottom run of shingles and drip edge off.
Yes, left as air gap. And, as noted, battens are not 2x4s. Supporting a metal roof is not a huge challenge, and think how much your cost goes up by using unnecessary 2xs when 1xs will more than suffice.
If your ceiling joists are insulated, do not add another separate layer of insulation. It can cause moisture, rot and mold.
