Insulation options for narrow ceiling space
The principle behind a flat roof is the same as with a sloped roof:  keep moisture out of the uninsulated space of the roof.  Most roof systems require venting. Venting is necessary to remove any moisture that comes from the inside of the home into the insulated space and causes condensation.  Condensation causes mold and other moisture problems. As long as there is enough room for air to move unimpeded throughout the space and in and out through the vents, you should be fine.  If you do not have the space for proper air movement, you can consider packing the entire roof with insulation. The idea with this method would be to prevent air movement in the entire space, preventing vapor from entering in the first place.  This method is used for vaulted ceilings and is fine if done right. You should do a super job of sealing the interior ceilings to prevent vapor transfer to that space. 

As far as recessed lighting is concerned, there are "cans" that are made for use in insulated spaces.  They have a temperature sensor that turns the fixture off if it gets too hot.