How does a priest heal in 25-mans?, Seeking Advanced Topics in Priest Play II Options

[Professor Frink]

A few years ago MongoJerry wrote a sticky topic "Advanced Topics in Priest Play" that laid out the basic rules of how to heal in a group environment--how much your heals do, which is appropriate to use in what situation, etc. It's been out of date for a long, long time now but I've had a hard time finding its equivalent for the endgame BC 25-man world.

Right now I'm playing a shadowpriest in a guild that's downed Illidan twice and is having great difficulty repeating the feat (8 hours of wipes over 2 days just ended in a no-kill this reset). We've always had high healer turnover, and right now we're rarely getting as many healers in as we need to do various encounters (which is too many), we're having trouble getting new recruits, and the healers we have are unreliable attendance-wise and less than stellar performance wise. I'd like to switch roles to become a healing priest, as we wouldn't have much trouble filling my DPS slot, and I'm getting a little bored of it anyway.

The problem is, I haven't healed anything seriously since doing the tier 0.5 upgrade questchain. I don't even know where to start, and I don't trust the healers we currently have to really know anything about how to play their class, and I can't find anything that looks like quality advice on the Internet. It seems that healing is usually learned through experience as you move through the game, but that's a luxury I don't have as we don't run anything but Hyjal and Black Temple.

So I'm wondering if anyone has information or references that would let me skip as much of the trial-and-error phase as possible. Specific things I can't seem to get straight answers on are:

* What stats do I need to be useful at all in T6-level content, +heal/regen/manabowl wise?
* How much +heal should I give up to get a point of Spirit, Int, Mp5, etc?
* What spells do I use? If I'm assigned to keep up a tank, should I be using flash or greater heal? Should I keep up a renew? What are the basic tradeoffs between heals now?
* Should I be using downranked spells? How do good healers go about deciding which downranks to use?
* Is queueing up and cancelling heals to stay inside the 5SR something I should be doing normally, or only when I'm desperate for mana?
* What non-25 man content, if any, would be the best practice for healing in Hyjal or BT?
* What do good endgame priests know that inexperinced priests don't even think to ask about?