1.
yes you will always need someone to peel for you, you have no way to escape crippling poison unless youre dwarf or shadow (both of which is tetchy anyway because cripp will get reapplied so fast). as far as heals, keep your shield on CD, pom on CD. Renew only if you can spare the global. otherwise try to fake a flash heal and get a full penance in, this way you dont waste a penance CD if trying to use it to fake. if you do manage to fake a kick, make sure that as youre casting your next full heal you continually turn to keep your back to the rogue so you can avoid the gouge
your main defensive cooldown, pain suppression, should be glyphed until you get good enough to be able to predict when theyre about to swap to you. when you use it depends alot on what class the other guy is. its important not to save it until youre 20% hp or something because youre going to either die anyway, or be so far behind on healing that you cant catch up again. I tend to use it early to keep control of the situation
2. main heals are penance, shield, pom, flash heal. secondary heals are renew and greater heal. if youre being focused then follow what I said in point 1. if youre healing someone else up then penance on CD, shield on CD, PoM if youre close to your partner (this way when it bounces to you, you can SW
to bounce it back to him). if your partner isnt going to die RIGHT NOW, then I like shield, penance, then greater heal (this will be hasted through your own haste and borrowed time down to like 1.6 seconds or so, assuming you have divine fury). renew is a nice thing to have on your partner when youre running to get a fear on someone. also if you see CC incoming on you, then I try to shield my partner before the CC hits, if he has weakened soul then I try to throw a renew/pom on him instead depending on how much hp he has at the time
3. playing mage/priest it is true that the priest is required to do an equal amount of WORK as the mage, but not really the same amount of DAMAGE. this means that as a priest you need to pick and choose your time to throw your burst out. youre not throwing out constant damage, youre throwing it out when you have a healer in a CC chain, your mage has a CS ready, your target has no defensive cooldowns to use etc. obviously you should throw out damage to help force some of those cooldowns but youll be doing burst dps, not consistent dps.
your main burst spells are holy fire and smite. For mage/priest I tend to spec into divine fury + searing light and glyph smite. you can also keep SW:P and Dev Plague on your target too if their team has no dispeller for those dots
your question about mindsear: I dont see alot of priests using it, but I find it great against rogue teams. say youve nuked a rogue quite hard and he blows his vanish and retreats. mind sear his healer to try to pop the rogue out of stealth again. This is really nice if their healer is a druid or a priest who are more mobile when they heal, because as they run around it increases the area of your aoe. Its also nice in openers to try to pop a rogue/druid out of stealth
IMO priests are quite an unforgiving class for beginners because 1) theyre squishy, 2) low mana regen, 3) small heals and 4) theyre really offensive healers. all of this, and especially point 4, requires alot of experience to know when you should heal, when you should dps, when you should dispel, how to switch from offensive to defensive and back again. But once you get the hang of it, we have ALOT of utility: off/def dispels, hard range nukes, mass dispel, mana burn. these things make us a versatile class but it requires alot of play to understand how and when to use them
oh and bind dispel magic to like every button on your keyboard, youll be using it, alot