Hi, Billy. And the answers are: (1) yes (2) no (3) that depends
You want to keep the warm air in your house in the winter. The heat and humidity wants to go thru your ceiling into the attic. So to lower your utility cost, add more insulation above the ceiling. This works just the same way with cold air from air conditioning in the summer. Heat wants to move to a colder location. You might check with your local utility company to see if they will give you an "energy audit". For free or at nominal cost, they will tell you how much you can save in energy cost buy adding insulation and check all of your house for other energy wasters as well.
The problem in the winter is the humidity. The water vapor goes up thru the ceiling, hits the cold and condenses. That's why you don't want to use faced insulation in the attic. If you stop the humidity from going up, it will still condense and the water will ruin your ceiling. So now we got humid air in the attic and the question is how to get it out. We don't have many slate roofs in Michigan so I can't tell you what to do except that you should get a slate roof installer out and go over it with him. There might be enough air gaps that condensation is not a problem. But around here, I see more and more roof ventilation being added. I'll bet that there are ridge vents that would be great for this but your local tile roof person will be your best source of information. Gable end vents are nowhere near as efficient at getting the moist air out as ridge vents. And I would do as little work on the tile roof myself as possible. This work is best left to the pro's.