While on the topic you mention going Disc / Disc which Is what we were going to try however it seems like having one Disc and one Holy would work the best with him instantly topping off the melee with Prayer of Healing and then CoH immediately following.
Disc/Disc isn't really an effective combo, for 25HM at least.
What we basically did, was designate 3 groups for myself (disc), a melee group for the Holy Priest (and left overs for the sham's CH bounce). Doesn't put out the max number of possible shields, but its a little more reliable (especially when 1 of those healers is picked up). I shield my 15, get a PoM out on one of the common targets to not remove infest (mages/locks/paladin's seem to take slightly more damage, possibly through a lack of % spell based damage reductions in talents), when done with those, I'll get a few renew's going (depending on time).
Those 15 players are mostly my responsibility, so when infest rolls out, those are my priorities to remove infest debuff (rather than restarting shielding immediately, with 2 people still having infest).
We typically use a Sham bouncing CH's through the MT (picking up the other melee in G1 for infest, along with CoH after the PoH cast for Holy Priest), keeping up 10% reduction. The priest version of this is much more difficult to manipulate on a single target (Target MT w/ 6 people at lower % Hp around him = those 6 get healed and not tank, doesn't 'have' to hit the primary target).
As for the rough damage at transition, the P1 transition is fairly low impact (as the platform is still there), and no one should be taking Winter ticks. But the second transition, everyone in the raid takes at least 1, plus your dealing with P+S. About the only thing you can do, is either have the raid fully shielded already (can be tricky, but if there's an infest right before transition, there needs to be a raid CD call). Also, having designated sides to run out at helps to (2 ranged run out about 10-15yds from the LK MT with a healer, few others and other healer on other side, spread far enough to maintain range, but avoid 10person cone).
If damage during the transition (melee heavy groups), only way I've seen thats really effective, its to just have melee watch out, split them to 2 groups, if they get a 4 stack (self monitored), each group takes 2-3 steps further apart (out of dps range), for 1 P+S pulse, which will reset the stacks on at least most of them.