According to the ARK Dev Kit, the damage on the DOT (Damage over time) buff applied is 9% damage to max health per second, for 6 seconds. Reapplying the buff will delete the old stack, and reset the buff timer, resulting in no benefit to applying the buff prior to the previous stack expiring. The Minimum damage is 12, and the maximum damage is 100. As most targets (dinosaurs) have over 1000 hp, this means it will most often do exactly 100 damage to tames, while doing variable damage to players.
Using the Flamethrower on Spike Walls, currently, still damage the player.
No, creatures killed with the Flamethrower do not give Cooked Meat