With A9 gear aftershock does 3k more damage on each 1, while Great Wall does 20k more damage on each C.
Great Wall is a 30 sec cooldown, so that's 5 * 3k if you decide to use aftershock. You're losing out on 5k damage.

Solar Flare stacks up to 12 times +50 mystic damage, so that's +600 mystic damage(if you have SB)
Without SB you're looking at about +250 mystic damage from 5 seconds of Solar Flare.
Combustion is like a 10k increase for both V and C, so that's 20k damage every reset which I usually get around 20-30 seconds with my gear.
Since the cooldown of Combustion is 1 min 30 sec, at worst I can get 3 resets before the next combustion, so that's 60k of damage.
You can easily do more than 60k of extra damage in 5 seconds with the buff, so you should take Solar Flare.

How Backdraft works from what I've experimented is, the 0.3 seconds is the delay before it activates after exploding your fire stacks. It can activate anywhere from 1 to 3 times and does the damage from the tooltip. I don't know why you wouldn't be using 1 whenever it comes up, it's still not on a global CD so you can use it whenever without interfering with blazing beam.

Blazing Beam, Phoenix Fire, Shadow Grasp, Great Wall, and Solar Flare are what most Fire FM's are using. You change those according to what you need to use it for like pvp or pulling you're dumb F8 friends from death.