There are two ways to look at it. 1) You're a healer, and if you keep people alive, then you're doing your job. If you don't feel comfortable dps'ing, then it's not required. 2) You should never be standing there doing nothing. You can start small. Apply your DoTs without cleric stance, then work your way up to switching to cleric and applying DoTs. Then once your comfortable with that you can add Stones in, and Holy.

The Mana thing isn't an issue unless you have a bunch of unnecessary raises. The only "long" fights are in raiding, and you'll have other members that can regenerate your mana.