Hi Jarod and welcome to the forum.
Personally, I would not convert a rolled metal roof to a shingle roof without a permit, which should require an engineering report. You may be able to find out if the existing structure is approved for sheathing and shingles from the manufacturer.
Whether you should vent the shingle roof or not is a very good question. Obviously, the venting would not be very beneficial to the "attic" area with the metal roof intact. But I also believe that nailing sheathing to the trusses, through the metal roof could cause condensation problems. There is also a question as to what would happen with the additional heat build-up caused by the shingles absorbing heat.
I would think that if your existing trusses are built to handle the weight of sheathing and a shingle roof, the building department would probably want you to remove the existing metal roof. Just guessing here really. But it would make more sense to me than sandwiching the metal roof between your existing vapor barrier and the shingle roof. If your trusses are acceptable, you would end up with a standard shingle roof installation.
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All information and advice given is for entertainment and informational purposes only. The person doing the work is solely responsible to insure that their work complies with their local building code and OSHA safety regulations.