1. Personally i find AST the most satisfying. In the end I see all the healers dps as 90% the same, DoTs then spam one skill until you need to reapply. What makes AST feel more satisfying to me is simply the fact the skills look cooler. WHM has the blandest looking skills in the game to me aside from holy and to a lesser extent aero 3 and SCH isn't far off. I think Malefic looks super cool in comparison and Gravity is by far the coolest looking(and sounding) skill in the game. Plus the benefit of tossing cards out on people makes it even more fun since aside from my own DPS I get to amplify my teammates as well.

2. Yes in fights especially bosses you should ideally never let your DoTs fall off. For trash mobs it's a little more lenient to allow depending on how close they are to death.