FG vs cellulose is not even a contest. Leave the FG at the store, and blow cellulose right over what is there. You are very wise to air seal; do so like a madman. I think the sheet rock boxes for the lights will work, but check w/ a code-freak on that. Ventilation should be 1 sf of vent to every 150-300 sf of attic area. If you have wind, keep soffit openings near the house, to prevent wind washing. Have a good weekend.