There's a 4th option, use a 21/41 (+9) Disc/Holy Priest. It gives you the benefit of Soul Warding and Circle of Healing; however this priest won't have Rapture, so mass-shielding the raid will not be sustainable. (You also lose Guardian Spirit and Pain Suppression/Power Infusion).

Also, every holy priest should be able to cover at least 6 people through Infest with no problem. Prayer of Mending should guarantee 1 person and a Prayer of Healing on melee should cover 5. The shield difference is surprisingly small, because the holy priest has more spellpower and couple of talents to help make-up for Borrowed Time.

I'd say you should spread out better. In 10-man there shouldn't be any big problem to spread your ranged so only 1 or 2 are going to be hit simultaneously. If you're melee heavy then the things look a bit worse but... well, I found that all or almost all ICC10 fights benefit ranged heavy group over melee anyway

Bobbeh,

On the subject of the shaman and druid assignment, keep in mind that you pretty much want 100% inspiration on tanks in this fight, since Soul Reaper gibs are still very dangerous and sadly neither the druid nor the paladin healers can't do provide this buff. The disc priest will be shielding raid and almost never the tanks, and the holy will also be busy with infest with it comes, leaving your tank sometimes without the -10% physical dmg buff.

With the tactic we use (Shaman/Holy) the 2 infest healers are essentially just tank healing anyway until the brief period where they need to clear infests meaning inspiration will more than likely be up all the time.

As for Soul Reaper our second tank just range taunts it so when done right it'll never result in a death.

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.

Holy with Body and Soul is very effective for Heroic Halion. If you have a Disc Priest, ask them to restrain themselves from rampant bubbling in the 1st phase to leave your raiders clear for a free sprint when they get consumption/combustion. It's also very mobile in the twilight realm. You are able to put out a lot of healing with your instants and keep Guardian Spirit for dangerous moments on your tank or to save people that get RNG'd with debuffs or cutter.

I used the 18/53/0 spec I'm wearing for the longevity with instants, and even got some good healing with Lightwell when I remembered to use it.

Halion 10 Heroic