In my opinion level Ret until 35 or 40 when you can get Recknoning maxed or Holy Shield. A great thing about being Protection is the ability to AoE grind. A good amount of a Paladin's damage and Aggro control comes from Spell Damage, seeing as how a lot of his attacks and Seals give Holy Damage, and Righteous Fury is fueled off of Holy Damage. As far as I know, only Crusader Strike is fueled off of Melee Damage, and that is the final Retribution talent.
And for your question to Solo Quest while Protection, yes you can. You take a good amount longer to kill something, but if you stack Judgment of Light and Seal of Light, you'll be fine because of how often it Procs. At level 40 I can fight up to 4 level 44 Gnolls in Feralas before I have to start to throw down heals and things of that sort. Also, make sure you get a LOT of Stamina. Stamina is your friend. Love it. Hold it. Console it when it's upset.