Sleeper fastening, Condensation, Underlay lapping

I'm about to install my first "Zincalume" metal roof, about 20square, on a section of my house. All the decking/sheathing is installed and covered with ice and water shield. I have the option to add 1x4 spruce sleepers on top of the underlay at a 45 degree angle fastened to the rafters (24" on center) and spaced about 18" apart. If I do so I believe ventilation will be improved and allow me to some day insulate the attic. But I have few concerns about how to best proceed.

1) Are bright common nails ok to fasten the sleepers? how about coated sundeck screws? or must I absolutely use galvanized nails?

2) Given the 3/4" air space above the underlay if I add the sleepers, does condensation (when the dew point and temperature meet, which may be affected by heat from inside the house?) have more or less of a chance of forming under the metal and running down to meet the sleepers and compromising them - given they must be untreated wood to maintain the integrity of the zincalume?

3) Is there a reason why the lapping of the underlay needs to be "at least 6"? Multiple people have told me this, but why? I think even 4" of merged and melted tar substance should be good enough, or?

4) Is it true that underlay breathes through the seams and overlaps? Then wouldn't less overlap make breathing easier?

Thanks for any and all insights into these particulars.

Regards,
Mark