Firstly how long do you spend getting the mages free after each flash freeze? Correct me if i'm wrong but i think Flash Freeze is on something like a 40-45 sec CD. So to gain real beenfit from any of the NPCs you need to break them out in like 1-2 seconds max.
Secondly the Toasty fire stacks a spellpower buff on Hodir, it stacks to 30 so initally you want to get it on him asap, so that's all the casters using quick spells to stack it. Once it's up then they can break into a normal rotation. Assuming your quick at releasing the mages after each flash freeze you can keep the buff up throughout the fight.
The crit buff just needs to be spread around your DPS who would benefit most from additional crit. Liberally applying it to the mages wouldn't hurt.
Having your tank use FR gear is a major plus, it really smoothes out the frozen blow damage and the healers can work at keeping the raid alive. This in turn allows your DPS to not have to use FR gear.