Frost mage talents

Hello,
I did some research but I couldn't find anything i'm looking for, and i'm trying to figure out frost mage's talent machanism.
So let me put this in an example, I cast frostbolt, Brain Freeze procs, I start casting frostbolt again, and instantly after casting is finished I cast a frostfire bolt (instant cast) which consumes the talent buff. The frostbolt still hits moments before the frostfire bolt, and if another Brain Freeze procs, in the time between frostbolt hitting the target and the frostfire bolt hitting it, the talent buff is consumed when the ffb hits the target having two buffs consumed for one ffb. Now this I think was fixed not so long ago (at least on Lordaeron). My question actually is, if when I cast the first frostbolt the talent that procs is Fingers of Frost, then when I cast the second one and immediately after it I cast Ice Lance, which should consume the buff, when should the buff be consumed? When I cast ice lance or when it hits the target? The talent itsself is making the spells act as if the target is frozen, so logic says that it should be consumed on target hit, and that's how it is (at least on Lordaeron). Now if that's not how it should be then mages are downed by a bit. In the time between second frostbolt hitting the target and ice lance hitting it, another FoF procs, and the first charge of FoF is just gone, which is okay, but, when ice lance hits the target it consumes one of the two charges of the second FoF proc leaving the mage with one FoF charge. If ice lance is to consume FoF on spell cast, one of the two charges of the first FoF proc will be consumed, meaning the second FoF proc is still full with two charges, leaving the mage with two instead of one FoF charge. Need an explanation...

you get proc when spell hit a target " because spell can miss, been resisted or absorbed and if this happen it doesnt count for proc"

and for buff part consume - it is consumed on spell cast