1) Currently at 90, I found at lower gear levels you don't have the mana to dps while keeping people up. Now though with higher gear (and other party members also having higher gear), I find myself dpsing more than healing, lol! So I would say as long as there is little healing required and you can sustain your mana while putting out some damage, feel free!
2) I personally use Healbot for healing, but both Vuhdo and Grid (plus Clique I believe) work well. As long as you can cast the heals on people without first manually clicking on them you will be able to get heals out faster. Get whichever addon you feel most comfortable with.
3) I don't pvp a bunch but have on occasion. Pretty much here you will rely on Holy shock, Word of Glory/Eternal Flame, and Flash of Light. I tend to put Beacon of Light on myself to keep a steady stream of healing on me while healing others. Use your Hand spells a lot! Hammer of Justice/Fist of Justice is very useful especially on those annoying melee hitting you). Denounce an enemy so they can't crit you or a group member also is helpful. Devotion aura (when you get it) or Bubble, if you want to be able to cast without worrying about being interrupted.
4) I leveled to 80 (during Wrath) mainly as holy while healing dungeons with a few quest thrown in as ret. Leveled to 85 (during Cata) as ret with dungeons as holy. I hated ret so decided to try leveling 85-90 as holy. Healers definitely have an easier time leveling outside of dungeons now than they did previously. I have second spec set up purely for dps as holy. Its all about the glyphs you pick. Glyph Holy Shock, Harsh Words and Denounce. Holy shock on CD, Harsh Words (Word of Glory) at 3hp and denounce to fill will do pretty well. Will be slower than ret but if you find you don't like ret its a good alternative.
Also, one thing i noticed is that since we have the cleanse ability which also dispels, is there any addon which alerts if we can dispel enemy buffs?
Our Cleanse only works to remove debuffs (magic, poison, disease) on friendly players and not enemy buffs. I like to use Decursive to show dispelable debuffs.