For a ridgevent to have full effect, it has to coincide with continous run soffit/eave venting. What type of venting does it currently have. If you haven't had any moisture/sagging issues, i would think it would be fine left alone. A lot has to do with geographic location as to what is the proper venting techniques used on a home.

The Owens Corning warranty requires "adequate ventilation" but does not say how to achieve that. Generally meeting typical building code standards would be considered adequate. If the house has an attic and it is ventilated by end wall vents that meet the building code requirements you should be OK.

Contrary to manufacturers' self-serving claims ventilation has little if anything to do with the longevity of asphalt shingles so your only concern should be the warranty.

Is it acceptable to put shingles over top an existing roof if there is only one layer there already?

Yes.

Depends on the condition of the old roof. It may also affect warranty of the shingles being installed.